## 1. Evolutionary Psychology: One research tradition among the various biological approaches to explaining human behavior



This entry focuses on the specific approach to evolutionary psychology
that is conventionally named by the capitalized phrase
"Evolutionary Psychology". This naming convention is David
Buller's (2000; 2005) idea. He introduces the convention to
distinguish a particular research tradition (Laudan 1977) from other
approaches to the biology of human
 behavior.[1]
 This research tradition is the focus here but lower case is used
throughout as no other types of evolutionary psychology are discussed.
Evolutionary psychology rests upon specific theoretical principles
(presented in section 2 below) not all of which are shared by others
working in the biology of human behavior (Laland & Brown 2002;
Brown et al. 2011). For example, human behavioral ecologists present
and defend explanatory hypotheses about human behavior that do not
appeal to psychological mechanisms (e.g., Hawkes 1990; Hrdy 1999).
Behavioral ecologists also believe that much of human behavior can be
explained by appealing to evolution while rejecting the idea held by
evolutionary psychologists that one period of our evolutionary history
is the source of all our important psychological adaptations (Irons
1998). Developmental psychobiologists take yet another approach: they
are anti-adaptationist. (Michel & Moore 1995; but see Bateson
& Martin 1999; Bjorklund & Hernandez Blasi 2005 for examples
of developmentalist work in an adaptationist vein.) These theorists
believe that much of our behavior can be explained without appealing
to a suite of specific psychological adaptations for that behavior.
Instead they emphasize the role of development in the production of
various human behavioral traits. From here on, "evolutionary
psychology" refers to a specific research tradition among the
many biological approaches to the study of human behavior.



Paul Griffiths argues that evolutionary psychology owes theoretical
debt to both sociobiology and ethology (Griffiths 2006; Griffiths
2008). Evolutionary psychologists acknowledge their debt to
sociobiology but point out that they add a dimension to sociobiology:
psychological mechanisms. Human behaviors are not a direct product of
natural selection but rather the product of psychological mechanisms
that were selected for. The relation to ethology here is that in the
nineteen fifties, ethologists proposed instincts or drives that
underlie our
 behavior;[2]
 evolutionary psychology's psychological mechanisms are the
correlates to instincts or drives. Evolutionary psychology is also
related to cognitive psychology and the cognitive sciences. The
psychological mechanisms they invoke are computational, sometimes
referred to as "Darwinian algorithms" or as
"computational modules". This overt cognitivism sets
evolutionary psychology apart from much work in the neurosciences and
from behavioral neuroendocrinology. In these fields internal
mechanisms are proposed in explanations of human behavior but they are
not construed in computational terms. David Marr's (e.g., 1983)
well known three part distinction is often invoked to distinguish the
levels at which researchers focus their attention in the cognitive and
neurosciences. Many neuroscientists and behavioral
neuroendocrinologists work at the implementation level while cognitive
psychologists work at the level of the computations that are
implemented at the neurobiological level (see Griffiths 2006).



Evolutionary psychologists sometimes present their approach as
potentially unifying, or providing a foundation for, all other work
that purports to explain human behavior (e.g., Tooby & Cosmides
1992). This claim has been met with strong skepticism by many social
scientists who see a role for a myriad of types of explanation of
human behavior, some of which are not reducible to biological
explanations of any sort. This discussion hangs on issues of
reductionism in the social sciences. (Little 1991 has a nice
introduction to these issues.) There are also reasons to believe that
evolutionary psychology neither unifies nor provides foundations for
closely neighboring fields such as behavioral ecology or developmental
psychobiology. (See the related discussion in Downes 2005.) In other
work, evolutionary psychologists present their approach as being
consistent with or compatible with neighboring approaches such as
behavioral ecology and developmental psychobiology. (See Buss's
introduction to Buss 2005.) The truth of this claim hangs on a careful
examination of the theoretical tenets of evolutionary psychology and
its neighboring fields. We now turn to evolutionary psychology's
theoretical tenets and revisit this discussion in section 4 below.


## 2. Evolutionary Psychology's Theory and Methods



Influential evolutionary psychologists, Leda Cosmides and John Tooby,
provide the following list of the field's theoretical tenets
(Tooby & Cosmides 2005):


1. The brain is a computer designed by natural selection to extract
information from the environment.
2. Individual human behavior is generated by this evolved computer
in response to information it extracts from the environment.
Understanding behavior requires articulating the cognitive programs
that generate the behavior.
3. The cognitive programs of the human brain are adaptations. They
exist because they produced behavior in our ancestors that enabled
them to survive and reproduce.
4. The cognitive programs of the human brain may not be adaptive
now; they were adaptive in ancestral environments.
5. Natural selection ensures that the brain is composed of many
different special purpose programs and not a domain general
architecture.
6. Describing the evolved computational architecture of our brains
"allows a systematic understanding of cultural and social
phenomena" (16-18).



Tenet 1 emphasizes the cognitivism that evolutionary psychologists are
committed to. 1 in combination with 2 directs our attention as
researchers not to parts of the brain but to the programs run by the
brain. It is these programs--psychological mechanisms--that
are products of natural selection. While they are products of natural
selection, and hence adaptations, these programs need not be currently
adaptive. Our behavior can be produced by underlying psychological
mechanisms that arose to respond to particular circumstances in our
ancestors' environments. Tenet 5 presents what is often called
the "massive modularity thesis" (see, e.g., Samuels 1998;
Samuels 2000). There is a lot packed into this tenet and we will
examine this thesis in some detail below in section 3. In brief,
evolutionary psychologists maintain that there is an analogy between
organs and psychological mechanisms or modules. Organs perform
specific functions well and are products of natural selection. There
are no general purpose organs, hearts pump blood and livers detoxify
the body. The same goes for psychological mechanisms; they arise as
responses to specific contingencies in the environment and are
selected for to the extent that they contribute to the survival and
reproduction of the organism. Just as there are no general purpose
organs, there are no general purpose psychological mechanisms.
Finally, tenet 6 introduces the reductionist or foundational vision of
evolutionary psychology, discussed above.



There are numerous examples of the kinds of mechanisms that are
hypothesized to underlie our behavior on the basis of research guided
by these theoretical tenets: the cheater-detection module; the
mind-reading module; the waist/hip ratio detection module; the snake
fear module and so on. A closer look at the waist/hip ratio detection
module illustrates the above theoretical tenets at work. Devendra
Singh (Singh 1993; Singh & Luis 1995) presents the waist/hip ratio
detection module as one of the suite of modules that underlies mate
selection in humans. This one is a specifically male psychological
mechanism. Men detect variations in waist/hip ratio in women.
Men's preferences are for women with waist/hip ratios closer to
.7. Singh claims that the detection and preference suite are
adaptations for choosing fertile mates. So our mate selection behavior
is explained in part by the underlying psychological mechanism for
waist/hip ratio preference that was selected for in earlier human
environments.



What is important to note about the research guided by these
theoretical tenets above is that all behavior is best explained in
terms of underlying psychological mechanisms that are adaptations for
solving a particular set of problems that humans faced at one time in
our ancestry. Also, evolutionary psychologists stress that the
mechanisms they focus on are universally distributed in humans and are
not susceptible to much, if any, variation. They maintain that the
mechanisms are a product of adaptation but are no longer under
selection (Tooby & Cosmides 2005, 39-40). Clark
Barrett's (2015) accessible and wide ranging introduction to
evolutionary psychology sustains this emphasis on evolved mechanisms
as the main focus of evolutionary psychology research. Barrett also
expands the scope of evolutionary psychology and notes the addition of
research methods developed since Cosmides and Tooby first set out the
parameters for research in the field. Some of Barrett's
proposals are discussed in sections 6 and 7 below. Todd Shackleford
and Viviana Weekes-Shackleford (2017) have just completed a huge
compendium of work in the evolutionarily based psychological sciences.
In this volume a vast array of different research methods are
presented and defended and there are a number of entries comparing the
merits of alternative approaches to evolutionary psychology.



The methods for testing hypotheses in evolutionary psychology come
mostly from psychology. For example, in Singh's work, male
subjects are presented with drawings of women with varying waist hip
ratios and ask to give their preference rankings. In Buss's work
supporting several hypothesized mate selection mechanisms, he
performed similar experiments on subjects, asking for their responses
to various questions about features of desired mates (Buss 1990).
Buss, Singh and other evolutionary psychologists emphasize the cross
cultural validity of their results, claiming consistency in responses
across a wide variety of human populations. (But see Yu & Shepard
1998; Gray et al. 2003 for alternate conflicting results to
Singh's.) For the most part standard psychological experimental
methods are used to test hypotheses in evolutionary psychology. This
has raised questions about the extent to which the evolutionary
component of evolutionary psychologists' hypotheses is being
tested (see, e.g., Shapiro & Epstein 1998; Lloyd 1999; Lloyd &
Feldman 2002). A response profile may be prevalent in a wide variety
of subject populations but this says nothing about whether or not the
response profile is a psychological mechanism that arose from a
particular selective regimen.


## 3. The Massive Modularity Hypothesis



Claims that the mind has a modular architecture, and even massively
modular architecture, are widespread in cognitive science (see, e.g.,
Hirshfield & Gelman 1994). The massive modularity thesis is first
and foremost a thesis about cognitive architecture. As defended by
evolutionary psychologists, the thesis is also about the source of our
cognitive architecture: the massively modular architecture is the
result of natural selection acting to produce each of the many modules
(see, e.g., Barrett & Kurzban 2006; Barrett 2012). Our cognitive
architecture is composed of computational devices, that are innate and
are adaptations (Samuels 1998; Samuels et al. 1999a; Samuels et al.
1999b; Samuels 2000). This massively modular architecture accounts for
all of our sophisticated behavior. Our successful navigation of the
world results from the action of one or more of our many modules.



Jerry Fodor was the first to mount a sustained philosophical defense
of modularity as a theory of cognitive architecture (Fodor 1983). His
modularity thesis is distinct from the massive modularity thesis in a
number of important ways. Fodor argued that our "input
systems" are modular--for example, components of our visual
system, our speech detection system and so on--these parts of our
mind are dedicated information processors, whose internal make-up is
inaccessible to other related processors. The modular detection
systems feed output to a central system, which is a kind of inference
engine. The central system, on Fodor's view is not modular.
Fodor presents a large number of arguments against the possibility of
modular central systems. For example, he argues that central systems,
to the extent that they engage in something like scientific
confirmation, are "Quinean" in that "the degree of
confirmation assigned to any given hypothesis is sensitive to
properties of the entire belief system" (Fodor 1983, 107). Fodor
draws a bleak conclusion about the status of cognitive science from
his examination of the character of central systems: cognitive science
is impossible. So on Fodor's view, the mind is partly modular
and the part of the mind that is modular provides some subject matter
for cognitive science.



A distinct thesis from Fodor's, the massive modularity thesis,
gets a sustained philosophical defense from Peter Carruthers (see
especially Carruthers 2006). Carruthers is well aware that Fodor (see
e.g. Fodor 2000) does not believe that central systems can be modular
but he presents arguments from evolutionary psychologists and others
that support the modularity thesis for the whole mind. Perhaps one of
the reasons that there is so much philosophical interest in
evolutionary psychology is that discussions about the status of the
massive modularity thesis are highly
 theoretical.[3]
 Both evolutionary psychologists and philosophers present and consider
arguments for and against the thesis rather than simply waiting until
the empirical results come in. Richard Samuels (1998) speculates that
argument rather than empirical data is relied on, because the various
competing modularity theses about central systems are hard to pull
apart empirically. Carruthers exemplifies this approach as he relies
heavily on arguments for massive modularity often at the expense of
specific empirical results that tell in favor of the thesis.



There are many arguments for the massive modularity thesis. Some are
based upon considerations about how evolution must have acted; some
are based on considerations about the nature of computation and some
are versions of the poverty of the stimulus argument first presented
by Chomsky in support of the existence of an innate universal grammar.
(See Cowie 1999 for a nice presentation of the structure of poverty of
the stimulus arguments.) Myriad versions of each of these arguments
appear in the literature and many arguments for massive modularity mix
and match components of each of the main strands of argumentation.
Here we review a version of each type of argument.



Carruthers presents a clear outline of the first type of argument
"the biological argument for massive modularity":
"(1) Biological systems are designed systems, constructed
incrementally. (2) Such systems, when complex, need to have massively
modular organization. (3) The human mind is a biological system and is
complex. (4) So the human mind will be massively modularly in its
organization" (Carruthers 2006, 25). An example of this argument
is to appeal to the functional decomposition of organisms into organs
"designed" for specific tasks, e.g. hearts, livers,
kidneys. Each of these organs arises as a result of natural selection
and the organs, acting together, contribute to the fitness of the
organism. The functional decomposition is driven by the response to
specific environmental stimuli. Rather than natural selection acting
to produce general purpose organs, each specific environmental
challenge is dealt with by a separate mechanism. All versions of this
argument are arguments from analogy, relying on the key transitional
premise that minds are a kind of biological system upon which natural
selection acts.



The second type of argument makes no appeal to biological
considerations whatsoever (although many evolutionary psychologists
give these arguments a biological twist). Call this the computational
argument, which unfolds as follows: minds are computational problem
solving devices; there are specific types of solutions to specific
types of problems; and so for minds to be (successful) general problem
solving devices, they must consist of collections of specific problem
solving devices, i.e. many computational modules. This type of
argument is structurally similar to the biological argument (as
Carruthers points out). The key idea is that there is no sense to the
idea of a general problem solver and that no headway can be made in
cognitive science without breaking down problems into their component
parts.



The third type of argument involves a generalization of
Chomsky's poverty of the stimulus argument for universal
grammar. Many evolutionary psychologists (see, e.g., Tooby &
Cosmides 1992) appeal to the idea that there is neither enough time,
nor enough available information, for any given human to learn from
scratch to successfully solve all of the problems that we face in the
world. This first consideration supports the conclusion that the
underlying mechanisms we use to solve the relevant problems are innate
(for evolutionary psychologists "innate" is usually
interchangeable with "product of natural
 selection"[4]).
 If we invoke this argument across the whole range of problem sets
that humans face and solve, we arrive at a huge set of innate
mechanisms that subserve our problem solving abilities, which is
another way of saying that we have a massively modular mind.



There are numerous responses to the many versions of each of these
types of arguments and many take on the massive modularity thesis head
on without considering a specific argument for it. I will defer
consideration of responses to the first argument type until section 4
below, which focuses on issues of the nature of evolution and natural
selection - topics in philosophy of biology.



The second type of argument is one side of a perennial debate in the
philosophy of cognitive science. Fodor (2000, 68) takes this argument
to rest on the unwarranted assumption that there is no
domain-independent criterion of cognitive success, which he thinks
requires an argument that evolutionary psychologists do not provide.
Samuels (see esp. Samuels 1998) responds to evolutionary psychologists
that arguments of this type do not sufficiently discriminate between a
conclusion about domain specific processing mechanisms and domain
specific knowledge or information. Samuels articulates what he calls
the "library model of cognition" in which there is domain
specific information or knowledge but domain general processing. The
library model of cognition is not massively modular in the relevant
sense but type two arguments support it. According to Samuels,
evolutionary psychologists need something more than this type of
argument to warrant their specific kind of conclusion about massive
modularity. Buller (2005) introduces further worries for this type of
argument by tackling the assumption that there can be no such thing as
a domain general problem solving mechanism. Buller worries that in
their attempt to support this claim, evolutionary psychologists fail
to adequately characterize a domain general problem solver. For
example, they fail to distinguish between a domain general problem
solver and a domain specific problem solver that is over generalized.
He offers the example of social learning as a domain general mechanism
that would produce domain specific solutions to problems. He uses a
nice biological analogy to drive this point home: the immune system is
a domain general system in that it allows the body to respond to a
wide variety of pathogens. While it is true that the immune system
produces domain specific responses to pathogens in the form of
specific antibodies, the antibodies are produced by one domain general
system. These and many other respondents conclude that type two
arguments do not adequately support the massive modularity thesis.



Fodor (2000) and Kim Sterelny (2003) provide different responses to
type three arguments. Fodor's response is that poverty of the
stimulus type arguments support conclusions about innateness but not
modularity and so these arguments can not be used to support the
massive modularity thesis. He argues that the domain specificity and
encapsulation of a mechanism and its innateness pull apart quite
clearly, allowing for "perfectly general learning
mechanisms" that are innate and "fully encapsulated
mechanisms" that are single stimulus specific and everything in
between. Sterelny responds to the generalizing move in type three
arguments. He takes language to be the exception rather than the rule
in the sense that while the postulation of an innate, domain specific
module may be warranted to account for our language abilities, much of
our other problem solving behavior can be accounted for without
postulating such modules (Sterelny 2003,
 200).[5]
 Sterelny's counter requires invoking alternate explanations for
our behavioral repertoire. For example, he accounts for folk
psychology and folk biology by appealing to environmental factors,
some of which are constructed by our forebears, that allow us to
perform sophisticated cognitive tasks. If we can account for our
success at various complex problem solving tasks, without appealing to
modules, then the massive modularity thesis is undercut. Sterelny
sharpens his response to massive modularity by adding more detail to
his accounts of how many of our uniquely human traits may have evolved
(see, e.g., Sterelny 2012). Sterelny introduces his "evolved
apprentice" model to account for the evolution of many human
traits that many assume require explanation in terms of massive
modularity, for example, forming moral judgments. Cecilia Heyes adopts
a similar approach to Sterenly in attacking massive modularity. Rather
than presenting arguments against massive modularity, she offers
alternative explanations of the development of folk psychology that do
not rely on the massive modularity thesis (Heyes 2014a; Heyes
2014b).



Heyes and Sterelny not only reject massive modularity but also have
little expectation that any modularity theses will bear fruit but
there are many critics of the massive modularity thesis who allow for
the possibility of some modularity of mind. Such critics of
evolutionary psychology do not reject the possibility of any kind of
modularity, they just reject the massive modularity thesis. There is
considerable debate about the status of the massive modularity thesis
and some of this debate centers around the characterization of
modules. If modules have all the characteristics that Fodor (1983)
first presented, then he may be right that central systems are not
modular. Both Carruthers (2006) and Barrett and Kurzban (2006) present
modified characterizations of modules, which they argue better serve
the massive modularity thesis. There is no agreement on a workable
characterization of modules for evolutionary psychology but there is
agreement on the somewhat benign thesis that "the language of
modularity affords useful conceptual groundwork in which productive
debates surrounding cognitive systems can be framed" (Barrett
and Kurzban 2006, 644).


## 4. Philosophy of biology vs. Evolutionary Psychology



Many philosophers have criticized evolutionary psychology. Most of
these critics are philosophers of biology who argue that the research
tradition suffers from an overly zealous form of
 adaptationism
 (Griffiths 1996; Richardson 1996; Grantham & Nichols 1999; Lloyd
1999; Richardson 2007), an untenable reductionism (Dupre 1999,
2001), a "bad empirical bet" about modules (Sterelny 1995;
Sterelny & Griffiths 1999; Sterelny 2003), a fast and loose
conception of fitness (Lloyd 1999; Lloyd & Feldman 2002); and most
of the above and much more (Buller 2005). (See also Downes
 2005.)[6]
 All of these philosophers share one version or other of
Buller's view: "I am unabashedly enthusiastic about
efforts to apply evolutionary theory to human psychology" (2005,
 x).[7]
 But if philosophers of biology are not skeptical of the fundamental
idea behind the project, as Buller's quote indicates, what are
they so critical of? What is at stake are differing views about how to
best characterize evolution and hence how to generate evolutionary
hypotheses and how to test evolutionary hypotheses. For evolutionary
psychologists, the most interesting contribution that evolutionary
theory makes is the explanation of apparent design in nature or the
explanation of the production of complex organs by appeal to natural
selection. Evolutionary psychologists generate evolutionary hypotheses
by first finding apparent design in the world, say in our
psychological make up, and then presenting a selective scenario that
would have led to the production of the trait that exhibits apparent
design. The hypotheses evolutionary psychologists generate, given that
they are usually hypotheses about our psychological capacities, are
tested by standard psychological methods. Philosophers of biology
challenge evolutionary psychologists on both of these points. I
introduce a few examples of criticisms in each of these two areas
below and then look at some responses to philosophical criticisms of
evolutionary psychology.



Adaptation is the one biological concept that is central to most
debates over evolutionary psychology. Every theoretical work on
evolutionary psychology presents the research tradition as being
primarily focused on psychological adaptations and goes on to give an
account of what adaptations are (see, e.g., Tooby & Cosmides 1992;
Buss et al. 1998; Simpson & Campbell 2005; Tooby & Cosmides
2005). Much of the philosophical criticism of evolutionary psychology
addresses its approach to adaptation or its form of adaptationism. Let
us quickly review the basics from the perspective of philosophy of
biology.



Here is how Elliott Sober defines an adaptation: "characteristic
*c* is an adaptation for doing task *t* in a population
if and only if members of the population now have *c* because,
ancestrally, there was selection for having *c* and *c*
conferred a fitness advantage because it performed task
*t*" (Sober 2000, 85). Sober makes a few further
clarifications of the notion of adaptation that are helpful. First, we
should distinguish between a trait that is *adaptive* and a
trait that is an *adaptation*. Any number of traits can be
adaptive without those traits being adaptations. A sea turtle's
forelegs are useful for digging in the sand to bury eggs but they are
not adaptations for nest building (Sober 2000, 85). Also, traits can
be adaptations without being currently adaptive for a given organism.
Vestigial organs such as our appendix or vestigial eyes in cave
dwelling organisms are examples of such traits (Sterelny and Griffiths
1999). Second, we should distinguish between ontogenic and
phylogenetic adaptations (Sober 2000, 86). The adaptations of interest
to evolutionary biologists are phylogenetic adaptations, which arise
over evolutionary time and impact the fitness of the organism.
Ontogenetic adaptations, including any behavior we learn in our
lifetimes, can be adaptive to the extent that an organism benefits
from them but they are not adaptations in the relevant sense. Finally,
adaptation and function are closely related terms. On one of the
prominent views of function--the etiological view of
functions--adaptation and function are more or less coextensive;
to ask for the function of an organ is to ask why it is present. On
the Cummins view of functions adaptation and function are not
coextensive, as on the Cummins view, to ask what an organ's
function is, is to ask what it does (Sober 2000, 86-87). (See
also Sterelny & Griffiths 1999, 220-224.)



Evolutionary psychologists focus on psychological adaptations. One
consistent theme in the theoretical work of evolutionary psychologists
is that "adaptations, the functional components of organisms,
are identified [...] by [...] evidence of their design: the
exquisite match between organism structure and environment"
(Hagen 2005, 148). The way in which psychological adaptations are
identified is by evolutionary functional analysis, which is a type of
reverse
 engineering.[8]
 "Reverse engineering is a process of figuring out the design of
a mechanism on the basis of an analysis of the tasks it performs.
Evolutionary functional analysis is a form of reverse engineering in
that it attempts to reconstruct the mind's design from an
analysis of the problems the mind must have evolved to solve"
(Buller 2005, 92). Many philosophers object to evolutionary
psychologists' over attribution of adaptations on the basis of
apparent design. Here some are following Gould and Lewontin's
(1979) lead when they worry that accounting for apparent design in
nature in terms of adaptation amounts to telling just-so stories but
they could just as easily cite Williams (1966), who also cautioned
against the over attribution of adaptation as an explanation for
biological traits. While it is true that evolutionary functional
analysis can lend itself to just-so story telling, this is not the
most interesting problem that confronts evolutionary psychology,
several other interesting problems have been identified. For example,
Elisabeth Lloyd (1999) derives a criticism of evolutionary psychology
from Gould and Lewontin's criticism of sociobiology, emphasizing
the point that evolutionary psychologists' adaptationism leads
them to ignore alternative evolutionary processes. Buller takes yet
another approach to evolutionary psychologists' adaptationism.
What lies behind Buller's criticisms of evolutionary
psychologists' adaptationism is a different view than theirs
about what is important in evolutionary thinking (Buller 2005). Buller
thinks that evolutionary psychologists overemphasize design and that
they make the contentious assumption that with respect to the traits
they are interested in, evolution is finished, rather than
ongoing.



Sober's definition of adaptation is not constrained only to
apply to organs or other traits that exhibit apparent design. Rather,
clutch size (in birds), schooling (in fish), leaf arrangement,
foraging strategies and all manner of traits can be adaptations (Seger
& Stubblefield 1996). Buller argues the more general point that
phenotypic plasticity of various types can be an adaptation, because
it arises in various organisms as a result of natural
 selection.[9]
 The difference here between Buller (and other philosophers and
biologists) and evolutionary psychologists is a difference in the
explanatory scope that they attribute to natural selection. For
evolutionary psychologists, the hallmark of natural selection is a
well functioning organ and for their critics, the results of natural
selection can be seen in an enormous range of traits ranging from the
specific apparent design features of organs to the most general
response profiles in behavior. According to Buller, this latter
approach opens up the range of possible evolutionary hypotheses that
can account for human behavior. Rather than being restricted to
accounting for our behavior in terms of the joint output of many
specific modular mechanisms, we can account for our behavior by
appealing to selection acting upon many different levels of traits.
This difference in emphasis on what is important in evolutionary
theory also is at the center of debates between evolutionary
psychologists and behavioral ecologists, who argue that behaviors,
rather than just the mechanisms that underlie them, can be adaptations
(Downes 2001). Further, this difference in emphasis is what leads to
the wide range of alternate evolutionary hypotheses that Sterelny
(Sterelny 2003) presents to explain human behavior. Given that
philosophers like Buller and Sterelny are adaptationists, they are not
critical of evolutionary psychologists' adaptationism. Rather,
they are critical of the narrow explanatory scope of the type of
adaptationism evolutionary psychologists adopt (see also Downes
2015).



Buller's criticism that evolutionary psychologists assume that
evolution is finished for the traits that they are interested in
connects worries about the understanding of evolutionary theory with
worries about the testing of evolutionary hypotheses. Here is Tooby
& Cosmides' clear statement of the assumption that Buller is
worried about: "evolutionary psychologists primarily explore the
design of the universal, evolved psychological and neural architecture
that we all share by virtue of being human. Evolutionary psychologists
are usually less interested in human characteristics that vary due to
genetic differences because they recognize that these differences are
unlikely to be evolved adaptations central to human nature. Of the
three kinds of characteristics that are found in the design of
organisms - adaptations, by-products, and noise - traits
caused by genetic variants are predominantly evolutionary noise, with
little adaptive significance, while complex adaptations are likely to
be universal in the species" (Tooby & Cosmides 2005, 39).
This line of thinking also captures evolutionary psychologists'
view of human nature: human nature is our collection of universally
shared adaptations. (See Downes & Machery 2013 for more discussion
of this and other, contrasting biologically based accounts of human
nature.) The problem here is that it is false to assume that
adaptations cannot be subject to variation. The underlying problem is
the constrained notion of adaptation. Adaptations are traits that
arise as a result of natural selection and not traits that exhibit
design and are universal in a given species (Seger & Stubblefield
1996). As a result, it is quite consistent to argue, as Buller does,
that many human traits may still be under selection and yet reasonably
be called adaptations. Finally, philosophers of biology have
articulated several different types of adaptationism (see, e.g.,
Godfrey-Smith 2001; Lewens 2009; Sober 2000). While some of these
types of adaptationism can be reasonably seen placing constraints on
how evolutionary research is carried out, Godfrey-Smith's
"explanatory adaptationism" is different in character
(Godfrey-Smith 2001). Explanatory adaptationism is the view that
apparent design is one of the big questions we face in explaining our
natural world and natural selection is the big (and only supportable)
answer to such a big question. Explanatory adaptationism is often
adopted by those who want to distinguish evolutionary thinking from
creationism or intelligent design and is the way evolutionary
psychologists often couch their work to distinguish it from their
colleagues in the broader social sciences. While explanatory
adaptationism does serve to distinguish evolutionary psychology from
such markedly different approaches to accounting for design in nature,
it does not place many clear constraints on the way in which
evolutionary explanations should be sought (Downes 2015). So far these
are disagreements that are located in differing views about the nature
and scope of evolutionary explanation but they have ramifications in
the discussion about hypothesis testing.



If the traits of interest to evolutionary psychologists are
universally distributed, then we should expect to find them in all
humans. This partly explains the stock that evolutionary psychologists
put in cross cultural psychological tests (see, e.g., Buss 1990). If
we find evidence for the trait in a huge cross section of humans, then
this supports our view that the trait is an adaptation --on the
assumption that adaptations are organ-like traits that are products of
natural selection but not subject to variation. But given the wider
scope view of evolution defended by philosophers of biology, this
method of testing seems wrong-headed as a test of an evolutionary
hypothesis. Certainly such testing can result in the very interesting
results that certain preference profiles are widely shared cross
culturally but the test does not speak to the evolutionary hypothesis
that the preferences are adaptations (Lloyd 1999; Buller 2005).



Another worry that critics have about evolutionary
psychologists' approach to hypothesis testing is that they give
insufficient weight to serious alternate hypotheses that fit the
relevant data. Buller dedicates several chapters of his book on
evolutionary psychology to an examination of hypothesis testing and
many of his criticisms center around the introduction of alternate
hypotheses that do as good a job, or a better job, of accounting for
the data. For example, he argues that the hypothesis of assortative
mating by status does a better job of accounting for some of
evolutionary psychologists' mate selection data than their
preferred high status preference hypothesis. This debate hangs on how
the empirical tests come out. The previous debate is more closely
connected to theoretical issues in philosophy of biology.



I said in my introduction that there is a broad consensus among
philosophers of science that evolutionary psychology is a deeply
flawed enterprise and some philosophers of biology continue to remind
us of this sentiment (see, e.g., Dupre 2012). However the
relevant consensus is not complete, there are some proponents of
evolutionary psychology among philosophers of science. One way of
defending evolutionary psychology is to rebut criticism. Edouard
Machery and Clark Barrett (2007) do just that in their sharply
critical review of Buller's book. Another way to defend
evolutionary psychology is to practice it (at least to the extent that
philosophers can, i.e. theoretically). This is what Robert Arp (2006)
does in a recent article. I briefly review both responses below.



Machery and Barrett (2007) argue that Buller has no clear critical
target as there is nothing to the idea that there is a research
tradition of evolutionary psychology that is distinct from the broader
enterprise of the evolutionary understanding of human behavior. They
argue that theoretical tenets and methods are shared by many in the
biology of human behavior. For example, many are adaptationists. But
as we saw above, evolutionary psychologists and behavioral ecologists
can both call themselves adaptationist but their particular approach
to adaptationism dictates the range of hypotheses that they can
generate, the range of traits that can be counted as adaptations and
impacts upon the way in which hypotheses are tested. Research
traditions can share some broad theoretical commitments and yet still
be distinct research traditions. Secondly, they argue against
Buller's view that past environments are not stable enough to
produce the kind of psychological adaptations that evolutionary
psychologists propose. They take this to be a claim that no
adaptations can arise from an evolutionary arms race situation, for
example, between predators and prey. But again, I think that the
disagreement here is over what counts as an adaptation. Buller does
not deny that adaptations-- traits that arise as a product of
natural selection--arise from all kinds of unstable environments.
What he denies is that organ-like, special purpose adaptations are the
likely result of such evolutionary scenarios. 



Arp (2006) defends a hypothesis about a kind of module--scenario
visualization--a psychological adaptation that arose in our
hominid history in response to the demands of tool making, such as
constructing spear throwing devices for hunting. Arp presents his
hypothesis in the context of demonstrating the superiority of his
approach to evolutionary psychology, which he calls "Narrow
Evolutionary Psychology," over "Broad Evolutionary
Psychology," with respect to accounting for archaeological
evidence and facts about our psychology. While Arp's hypothesis
is innovative and interesting, he by no means defends it conclusively.
This is partly because his strategy is to compare his hypothesis with
archaeologist Steven Mithen's (1996) non-modular
"cognitive fluidity" hypothesis that is proposed to
account for the same data. The problem here is that Mithen's
view is only one of the many alternative, evolutionary explanations of
human tool making behavior. While Arp's modular thesis may be
superior to Mithen's, he has not compared it to Sterelny's
(2003; 2012) account of tool making and tool use or to Boyd and
Richerson's (see, e.g., 2005) account and hence not ruled these
accounts out as plausible alternatives. As neither of these
alternative accounts rely on the postulation of psychological modules,
evolutionary psychology is not adequately defended.


## 5. Moral Psychology and Evolutionary Psychology



Many philosophers who work on moral psychology understand that their
topic is empirically constrained. Philosophers take two main
approaches to using empirical results in moral psychology. One is to
use empirical results (and empirically based theories from psychology)
to criticize philosophical accounts of moral psychology (see, e.g.,
Doris 2002) and one is to generate (and, in the experimental
philosophy tradition, to test) hypotheses about our moral psychology
(see, e.g., Nichols 2004). For those who think that some (or all) of
our moral psychology is based in innate capacities, evolutionary
psychology is a good source of empirical results and empirically based
theory. One account of the make-up of our moral psychology follows
from the massive modularity account of the architecture of the mind.
Our moral judgments are a product of domain specific psychological
modules that are adaptations and arose in our hominid forebears in
response to contingencies in our (mostly) social environments. This
position is currently widely discussed by philosophers working in
moral psychology. An example of this discussion follows.



Cosmides (see, e.g., 1989) defends a hypothesis in evolutionary
psychology that we have a cheater-detection
 module.[10]
 This module is hypothesized to underlie important components of our
behavior in moral domains and fits with the massively modular view of
our psychology in general. Cosmides (along with Tooby) argues that
cheating is a violation of a particular kind of conditional rule that
goes along with a social contract. Social exchange is a system of
cooperation for mutual benefit and cheaters violate the social
contract that governs social exchange (Cosmides & Tooby 2005). The
selection pressure for a dedicated cheater-detection module is the
presence of cheaters in the social world. The cheater-detection module
is an adaptation that arose in response to cheaters. The
cheater-detection hypothesis has been the focus of a huge amount of
critical discussion. Cosmides and Tooby (2008) defend the idea that
cheat detection is modular over hypotheses that more general rules of
inference are involved in the kind of reasoning behind cheater
detection against critics Ron Mallon (2008) and Fodor (2008). Some
criticism of the cheater-detection hypothesis involves rehashing
criticisms of massive modularity in general and some treats the
hypothesis as a contribution to moral psychology and invokes different
considerations. For example, Mallon (2008) worries about the coherence
of abandoning a domain general conception of ought in our conception
of our moral psychology. This discussion is also ongoing. (See, e.g.,
Sterelny 2012 for a selection of alternate, non-modular explanations
of aspects of our moral psychology.)


## 6. Human Nature



Evolutionary psychology is well suited to providing an account of
human nature. As noted above (Section 1), evolutionary psychology owes
a theoretical debt to human sociobiology. E.O. Wilson took human
sociobiology to provide us with an account of human nature (1978). For
Wilson human nature is the collection of universal human behavioral
repertoires and these behavioral repertoires are best understood as
being products of natural selection. Evolutionary psychologists argue
that human nature is not a collection of universal human behavioral
repertoires but rather the universal psychological mechanisms
underlying these behaviors (Tooby & Cosmides 1990). These
universal psychological mechanisms are products of natural selection,
as we saw in Section 2. above. Tooby and Cosmides put this claim as
follows: "the concept of human nature is based on a
species-typical collection of complex psychological adaptations"
(1990, 17). So, for evolutionary psychologists, "human nature
consists of a set of psychological adaptations that are presumed to be
universal among, and unique to, human beings" (Buller 2005,
423). Machery's (2008) nomological account of human nature is
based on, and very similar to, the evolutionary psychologists'
account. Machery says that "human nature is the set of
properties that humans tend to possess as a result of the evolution of
their species" (2008, 323). While Machery's account
appeals to traits that have evolved and are universal (common to all
humans), it is not limited to psychological mechanisms. For example,
he thinks of bi-pedalism as part of the human nature trait cluster.
Machery's view captures elements of both the sociobiological
view and the evolutionary psychology view of human nature. He shares
the idea that a trait must be a product of evolution, rather than say
social learning or enculturation, with both these accounts.



Some critical challenges to evolutionary psychological accounts of
human nature (and the nomological account) derive from similar
concerns as those driving criticism of evolutionary psychology in
general. In Section 4. we see that discussions of evolutionary
psychology are founded on disagreements about how adaptation should be
characterized and disagreements about the role of variation in
evolution. Some critics charge evolutionary psychologists of assuming
that adaptation cannot sustain variation. Buller's (2005)
criticism of evolutionary psychologists' account of human nature
also invokes variation (Here he follows Hull 1986 and Sober 1980). The
idea here is that humans, like all organisms, exhibit a great deal of
variation, including morphological, physiological, behavioral and
cultural variation (see also Amundson 2000). Buller argues that the
evolutionary psychology account of human nature either ignores or
fails to account for all of this variation (see also Lewens 2015 and
Ramsey 2013). Any account that restricts human nature to just those
traits we have in common and which also are not subject to change,
cannot account for human variation.



Buller's (2005) criticism of evolutionary psychologists'
notion of human nature (or the nomological account) is based on the
idea that we vary across many dimensions and an account of human
nature based on fixed, universal traits cannot account for any of this
variation. The idea that to account for human nature, we must account
for human variation is presented and defended by evolutionary
psychologists (see, e.g., Barrett 2015), anthropologists (see e.g.
Cashdan 2013) and philosophers (see, e.g., Griffiths 2011 and Ramsey
2013). Barrett agrees with Buller (and others) that evolutionary
psychologists have failed to account for human variation in their
account of human nature. Rather than seeing this challenge as a knock
down of the whole enterprise of accounting for human nature, Barrett
sees this as a challenge for an account of human nature. Barrett says
"Whatever human nature is, it's a biological phenomenon
with all that implies" (2015, 321). So, human nature is "a
big wobbly cloud that is different from the population clouds of
squirrels and palm trees. To understand human minds and behaviors, we
need to understand the properties of our own cloud, as messy as it
might be" (2015, 232). Rather than human nature being a
collection of shared fixed universal psychological traits, for
Barrett, human nature is the whole human trait cluster, including all
of the variation in all of our traits. This approach to human nature
is sharply different than the approach defended by either Wilson,
Tooby and Cosmides or Machery but is also subject to a number of
criticisms. The main thrust of the criticisms is that such a view
cannot be explanatory and is instead merely a big list of all the
properties that humans have had and can have (see, e.g., Buller 2005;
Downes 2016; Futuyma 1998; and Lewens 2015). Discussion over the
tension between evolutionary psychologists' views and the
manifest variation in human traits continues in many areas that
evolutionary psychologists focus on. Another example of this broader
discussion is included in Section 7. below.


## 7. Applications of Evolutionary Psychology and Prospects for Further Debate



Evolutionary psychology is invoked in a wide range of areas of study,
for example, in English Literature, Consumer Studies and Law. (See
Buss 2005 for discussion of Literature and Law and Saad 2007 for a
detailed presentation of evolutionary psychology and consumer
studies.) In these contexts, evolutionary psychology is usually
introduced as providing resources for practitioners, which will
advance the relevant field. Philosophers have responded critically to
some of these applications of evolutionary psychology. One concern is
that often evolutionary psychology is conflated with evolution or
evolutionary theory in general (see, e.g., Leiter & Weisberg 2009
and Downes 2013). The discussion reviewed in Section 4. above, reveals
a good deal of disagreement between evolutionary theorists and
evolutionary psychologists over the proper account of evolution.
Evolutionary psychologists offer to enhance fields such as Law and
Consumer Studies by introducing evolutionary ideas but what is in fact
offered is a selection of theoretical resources championed only by
proponents of a specific approach to evolutionary psychology. For
example, Gad Saad (2007) argues that Consumer Studies will profit
greatly from the addition of adaptive thinking, i.e. looking for
apparent design, and by introducing hypothetical evolved modules to
account for consumer behavior. However, this does not appear to be an
effort to bring evolutionary theory, broadly construed, to bear on
Consumer Studies (Downes 2013). Promoting disputed theoretical ideas
is certainly problematic but bigger worries arise when thoroughly
discredited work is promoted in the effort to apply evolutionary
psychology. Owen Jones (see, e.g., 2000; 2005), who believes that Law
will benefit from the application of evolutionary psychology,
champions Randy Thornhill and Craig Palmer's (2000) widely
discredited view that rape is an adaptation as exemplary evolutionary
work (see de Waal 2000, Coyne & Berry 2000, Coyne 2003, Lloyd
2003, Vickers & Kitcher 2003, and Kimmel 2003). Further, Jones
(2000) claims that the critics of Thornhill and Palmer's work
have no credibility as scientists and evolutionary theorists. This
claim indicates Jones' serious disconnect with the wider
scientific (and philosophical) literature on evolutionary theory
(Leiter & Weisberg 2009).



Aside from monitoring the expansion efforts of evolutionary
psychology, there are a number of other areas in which further
philosophical work on evolutionary psychology will be fruitful. The
examples given above of work in moral psychology barely scratch the
surface of this rapidly developing field. There are huge numbers of
empirical hypotheses that bear on our conception of our moral
psychology that demand philosophical scrutiny. (Hauser 2006 includes a
survey of a wide range of such hypotheses.) Also, work on moral
psychology and the emotions can be drawn together via work on
evolutionary psychology and related fields. Griffiths (1997) directed
philosophical attention to evolution and the emotions and this kind of
work has been brought into closer contact with moral psychology by
Nichols (see, e.g., his 2004). In philosophy of mind there is still
much that can be done on the topic of modules. Work on integrating
biological and psychological concepts of modules is one avenue that is
being pursued and could be fruitfully pursued further (see, e.g.,
Barrett & Kurzban 2006; Carruthers 2006) and work on connecting
biology to psychology via genetics is another promising area (see e.g.
Marcus 2004). In philosophy of science, I have no doubt that many more
criticisms of evolutionary psychology will be presented but a
relatively underdeveloped area of philosophical research is on the
relations among all of the various, theoretically different,
approaches to the biology of human behavior (but see Downes 2005;
Griffiths 2008; and Brown et al. 2011). Evolutionary psychologists
present their work alongside the work of behavioral ecologists,
developmental psychobiologists and others (see, e.g., Buss 2005; Buss
2007) but do not adequately confront the theoretical difficulties that
face an integrated enterprise in the biology of human behavior.
Finally, while debate rages between biologically influenced and other
social scientists, most philosophers have not paid much attention to
potential integration of evolutionary psychology into the broader
interdisciplinary study of society and culture (but see Mallon and
Stich 2000 on evolutionary psychology and constructivism). In
contrast, feminist philosophers have paid attention to this
integration issue as well as offered feminist critiques of
evolutionary psychology (see Fehr 2012, Meynell 2012 and the entry on
 feminist philosophy of biology).
 Gillian Barker (2015), shares some evolutionarily based criticisms of
evolutionary psychology with philosophers of biology discussed in
Section 4. but also assesses evolutionary psychology in relation to
other social sciences. She also adds a novel critical appraisal of
evolutionary psychology. She argues that, as currently practiced,
evolutionary psychology is not a fruitful guide to social policy
regarding human flourishing.



The publication of Shackleford and Weekes-Shackleford's (2017)
huge collection of articles on issues arising in the evolutionary
psychological sciences provides a great resource for philosophers
looking for material to fuel critical discussion. Many evolutionary
psychologists are aware of the difficulty variation presents for some
established approaches in their field. This issue confronts those
interested in developing accounts of human nature, as noted above
(Section 6.), but also arises when confronting many of the varying
human behaviors evolutionary psychologists seek to account for. For
example, human aggression varies along many dimensions and confronting
and accounting for each of these types of variation is a challenge for
many evolutionary psychologists (Downes & Tabery 2017). Given that
evolutionary psychology is just one, among many, evolutionarily based
approaches to explaining human behavior, the most promising critical
discussions of evolutionary psychology should continue to come from
work that compares hypotheses drawn from evolutionary psychology with
hypotheses drawn from other evolutionary approaches and other
approaches in the social sciences more broadly construed. Stephan
Linquist (2016) takes this approach to evolutionary
psychologists' work on cultures of honor. Linquist introduces
hypotheses from cultural evolution that appear to offer more
explanatory bite than those from evolutionary psychology. The broader
issue of tension between evolutionary psychology and cultural
evolution here will doubtless continue to attract the critical
attention of philosophers. (See Lewens 2015 for a nice clear
introduction to and discussion of alternative approaches to cultural
evolution.)



Interest has re-emerged in the relation(s) between evolutionary
psychology and the other social sciences (Buss 2020). Some time ago,
John Dupre (1994) diagnosed evolutionary psychology as an
exercise in scientific imperialism. Dupre later characterized
scientific imperialism as "the tendency for a successful
scientific idea to be applied far beyond its original home, and
generally with decreasing success the more its application is
expanded" (2001, 16). Dupre uses "scientific
imperialism" in a pejorative sense and marshals this as a
criticism of evolutionary psychology. (See Downes 2017 for further
discussion of scientific imperialism and evolutionary psychology.)
Buss (2020) does not cite Dupre but might well be responding to
him when he proposes that evolutionary psychology constitutes a
scientific revolution in Kuhn's sense. Buss argues that
evolutionary psychology is superior to other approaches in psychology,
because it has supplanted them (or at least should supplant them) just
as Einstein's physics supplanted Newton's or just as
cognitive psychology supplanted behaviorism. (David Reich [2018] casts
ancient DNA research in similarly Kuhnian terms and offers it up as
superior to all previous approaches in archaeology.) Buss takes
evolutionary psychology to be a meta-theoretical approach best fit for
guiding all of psychology. This is one of the many ways in which his
appeal to Kuhn is strained, as Buss is not looking back on the
supplanting of one theoretical framework by another but rather arguing
for the superiority of his approach to others available in psychology.
A further, and quite specific way that Buss sees evolutionary
psychology as superior to other approaches in psychology (and the
social sciences in general, is that evolutionary psychology ignores
(or should ignore) proximate explanations. For Buss, evolutionary
psychology offers ultimate explanations and these are enough. However,
many areas of biology, for example, physiology, trade in proximate
explanations and are not likely to be cast aside because of this
focus. This implies that there is still a place for proximate
explanations in psychology. This brief discussion indicates that the
relations between evolutionary psychology and the rest of psychology,
and the social sciences, more broadly is a topic well worth pursuing
by philosophers of science and Buss' and Dupre's
accounts present interesting alternate starting points in this
endeavor.



Finally, philosophers of science will doubtless continue to check the
credentials of evolutionary ideas imported into other areas of
philosophy. Philosophers of biology in particular, still voice
suspicion if philosophers borrow their evolutionary ideas from
evolutionary psychology rather than evolutionary biology. Philip
Kitcher (2017) voices this concern with regards to Sharon
Street's (2006) appeals to evolution. Kitcher worries that
Street does not rely on "what is known about human
evolution" (2017, 187) to provide an account of how her traits
of interest may have emerged. As noted above, Machery's
nomological notion of human nature (2008; 2017) is criticized on the
grounds that he takes his idea of an evolved trait from evolutionary
psychology as opposed to evolutionary biology. Barker (2015) also
encourages philosophers, as well as social scientists, to draw from
the huge range of theoretical resources evolutionary biologists have
to offer, rather than just from those provided by evolutionary
psychologists.