and the type of love people experience shifts from passionate to companionate. In 1986, Robert Sternberg suggested that there are actually three components of love: intimacy, passion, and commitment. When two (or more) people experience all three, they are said to be in a state of consummate love. According to social exchange theory, relationships are based on rational choice and cost-benefit analysis. A person may leave a relationship if their partner's "costs" begin to outweigh their benefits, especially if there are good alternatives available. This theory is similar to the minimax principle proposed by mathematicians and economists. With time, long-term relationships tend to become communal rather than simply based on exchange. Research Methods Social psychology is an empirical science that attempts to answer questions about human behavior by testing hypotheses. Careful attention to research design, sampling, and statistical analysis is important in social psychology. Whenever possible, social psychologists rely on controlled experimentation, which requires the manipulation of one or more independent variables in order to examine the effect on a dependent variable. Experiments are useful in social psychology because they are high in internal validity, meaning that they are free from the influence of confounding or extraneous variables, and so are more likely to accurately indicate a causal relationship. However, the small samples used in controlled experiments are typically low in external validity, or the degree to which the results can be generalized to the larger population. There is usually a trade-off between experimental control (internal validity) and being able to generalize to the population (external validity). Because it is usually impossible to test everyone, research tends to be conducted on a sample of persons from the wider population. Social psychologists frequently use survey research when they are interested in results that are high in external validity. Surveys use various forms of random sampling to obtain a sample of respondents that is representative of a population. This type of research is usually descriptive or correlational because there is no experimental control over variables. Some psychologists have raised concerns for social psychological research relying too heavily on studies conducted on university undergraduates in academic settings, or participants from crowdsourcing labor markets such as Amazon Mechanical Turk. In a 1986 study by David O. Sears, over 70% of experiments used North American undergraduates as subjects, a subset of the population that is unrepresentative of the population as a whole. Regardless of which