the senses play a pivotal role in the sexual response of women . in basson s model , emotional intimacy with a partner may render women sexually aroused through specific stimuli , of which physical contact ( the tactile sense ) is pre - eminent ( basson , 2001 ) . developed the dual - control ( excitation / inhibition ) model of the sexual response and emphasized its neurophysiologic substrate ( bancroft et al . tactile stimuli give excitatory ( or inhibitory ) input to the spinal cord , thereby mediating the genital arousal response . while tactile interaction outside a one - to - one relationship is frowned upon , no one in our image - obsessed world doubts the role of visual stimuli in the sexual response . auditory stimuli , if less evident ( eg , lounge music ? ) , may equally act as sexual cues . but what about olfactory stimuli ? while most people would agree that unpleasant smells can be strong inhibitory signals ( in the bancroft sense ) , it is unclear whether olfactory cues can act as excitatory sexual input . the animal kingdom is full of examples of social and sexual chemosignals which have been given the name  pheromones  in 1959 . they are defined as  substances which are secreted to the outside by an individual and received by a second individual of the same species , in which they release a specific reaction , for example , a definite behavior or a developmental process  . over time , four types of pheromones have been postulated ( table 1 ) . releaser pheromones often prompt an immediate behavioral reaction , and have received the greatest attention in animals . for example , boars exude androstenone ( 5-androst-16-en-3-one ) which elicits lordosis ( mating readiness ) in sows during their estrus period ( dorries et al . , 1997 ) ; signaler pheromones are mainly social chemosignals : they provide information on the hierarchy and the animal s place in it , and on the type of food recently consumed by other animals and thus the nearby availability of food . while these effects are difficult to gauge in animals , they are the focus of most human studies . finally , primer pheromones affect ( neuro)-endocrine parameters , including the menstrual cycle . since endocrine changes generally modulate neuropsychological parameters , modulator and primer pheromones likely overlap . pheromones in humans may be present in bodily secretions such as urine , semen or vaginal secretions , breast milk and potentially also saliva and breath , yet most attention thus far has been directed toward axillary sweat . the axillary secretions originate from the highly dense eccrine and apocrine ( which secrete into hair follicles ) sweat glands and sebaceous glands . the secretions are odorless but apocrine sweat acquires odor after interaction with the cutaneous bacterial microflora . odor dispersal is modulated by several factors including clothing layers , axillary temperature , total hair surface , arm movements and the proximity of one s nose . the main components of sweat are simple organic acids ( eg , e-3-methyl-2-hexenoic acid and 3-methyl-3-hydroxylhexanoic acid ) ( hays , 2003 ) . the putative axillary sweat pheromones are steroid - structure components , in particular the odorous 16-androstenes : androstadienone ( 4,16-androstadien-3-one ) , androstenone ( mentioned above ) and androstenol ( 5-androst-16-en-3-ol ) ( fig . 1 ) . of these steroids , androstadienone appears to have the most robust effects upon both sexes but particularly upon women ( jacob et al . , 2001a ) . the sex gradient may be the result of the threefold difference in plasma levels as well as different skin flora . although these steroids are present at concentrations 50 - 100 times below those of some organic acids mentioned previously , their effect ultimately depends on the volatility and the perceptive threshold for these compounds as well as the duration of exposure . women tend to be more sensitive to androstadienone than men , and there appears to be a subpopulation of individuals with a high sensitivity (  super - smellers  ) 1 ) which was first isolated from the urine of pregnant women ( thysen et al . , 1968 ) . this chemosignal is likely present in other secretions such as axillary sweat . in general , as we will see , the effects of estratetraenol are smaller than those of androstadienone but go in the same direction . while in theory a pheromone targeted at women s partners , estratetraenol also has discernible effects in heterosexual women ( jacob et al . in small mammals , the presence of a lactating dam within the same cage induces maternal behavior and lengthens the cycle of the non - pregnant females ( reviewed in spencer et al . , 2004 ) . this hormone is important for parent - infant bonding and human attachment in general , and is therefore known as a  pro - social hormone  . oxytocin is readily detected in plasma but is also present , albeit at low concentrations , in urine and saliva ( feldman et al . the hormone is released bytactile interaction ( suckling but also , for example , massage ) and within - family conversation   auditory massage non - human mammals and non - mammal animal species like reptiles and amphibians exhibit a specific tissue that detects most pheromones : the vomeronasal organ ( vno ) , situated in the rostral ( anterior ) nasal cavity . the vno contains bipolar receptor cells , and its axonal nerve projections traverse the nasal septum and cross the cribriform plate to terminate in the accessory olfactory bulb , adjacent to - yet unconnected with- the main olfactory bulb ( fig . while the human fetus shows a vno with the typical bipolar cells and axonal projections , most evidence shows that it is functionally inactive after birth . genes coding for receptor proteins are pseudogenes with no protein expression ; bipolar receptor cells are no longer detectable , and the accessory olfactory bulb is absent . replace the degenerated vno - nerves as they migrate from the fetal vno - area to the basal forebrain ( wysocki and preti , 2004 ) . for example , the sow response to boar androstenone , alluded to above , was still present after the vno was blocked with surgical cement ( dorries et al . , 1997 ) . hence , the absence of a functional vno in human adults does not preclude pheromone perceptive abilities . human olfaction is as good as that of other mammals but it is consciously underdeveloped and overwhelmingly rated as the least important sense ( sela and sobel , 2010 )  people shamelessly gawp and overlisten but most balk at sniffing . pheromone detection can improve with practice , as was shown for androstenone ( wysocki et al . , 1989 ) . given that they affect physical parameters including the autonomic nervous system , bensafi et al . the study subjects were usually volunteers recruited by posters etc . , which means an overall younger and well - educated sample . in the majority of studies , care was taken to not disclose the true purpose of the study to the participants , who were told it was a study on  odors  . yet , as some groups built up an extensive research expertise , one might doubt that all campus volunteers were oblivious to the nature of the study ; inclusion restrictions ( such as women not using contraceptive pills ) and sexual preference disclosure might also have raised doubts that it was simply a study on  odors  . in the very best of studies , the study purpose was not revealed at all but the volunteers were  deceived  with some trick and the odor was subliminally introduced ( reviewed in havlicek and roberts , 2009 ) . in the majority of studies , the pheromone and control solutions were applied to the skin between the upper lip and nose on a cotton swab , taped gauze or q - tip , or they were sniffed from jars . clearly , the concentration of the pheromone in the test solution was pharmacological in all studies , typically 0.25 - 6.25 * 10 - 3mol / l ( compared to the physiological 0.44 * 10 - 6mol / l in apocrine sweat ) for androstadienone . given the high concentration and the odorous nature of 16-androstenes , a subset of the study participants could identify the musky smell of the pheromone solution , comparing it to  sweat  ,  urine  ,  a male roommate  or  clothes  ( jacob and mcclintock , 2000 ; lundstrm et al . , 2003a ) . therefore , in subsequent studies , researchers were at pains to mask the pheromone ( and control ) odor , for example by adding1% clove oil or 1% eugenol to the vehicle solution . in addition , masking odors do not eliminate bias : some participants thought the clove solutions smelled like christmas   with possible positive or negative associations ( lundstrm et al . , 2003a ) ; and some still ascribed a particular smell to the masked pheromone solution ( saxton et al . the pharmacological concentration of the pheromone is not in itself an issue  provided the experiments yield clear and consistent results with a convincing p level . another crucial issue in research regarding social / psychosexual chemosignals is the setting of the experiment and the gender ( and one might add the looks , voice and smell ) of the study attendant who distributes the solutions and supervises the tests . one study examined pheromones released by breastfeeding women using within - brassiere pads ( likely containing spilled milk , apocrine sweat , baby saliva , etc . ) and axillary pads obtained from donors , subsequently  equilibrated  by buffer solutions and cut into four . recipient women needed to wipe one specimen of both type of pads under their noses , each day for 3 months ( spencer et al . several studies with slightly different methodology demonstrated that female participants showed increased positive - stimulated mood ( affect ) and reduced negative mood after exposure to either male axillary extracts ( preti et al . , 2003 ) or purified androstadienone ( but not androstenol or vehicle ) ( jacob and mcclintock , 2000 ; jacob et al . the androstadienone - induced mood uplift was dependent on its dose ( bensafi et al . , 2004b ) and on the pre - exposure mood engendered by a video ( bensafi et al . , 2004a ) , and was abolished when a pain stimulus was given ( villemure and bushnell , 2007 ) . a robust reduction in feelings of negative affect and negative character was also found when androstadienone was administered by a vapor pulse directly to the putative vno ( grosser et al . , 2000 ) . in addition , female participants also felt more focused ( physically more aroused ) after exposure to androstadienone ( lundstrm et al . the improvement in mood and focus , induced by androstadienone , is potentially important for women s sexuality . indeed , studies in a laboratory setting showed that adoption of a positive mood ( ter kuile et al . , 2010 ) or a positive sexual schema ( set of cognitive views ) ( kuffel and heiman , 2006 ) facilitated subsequent mental and genital arousal , also in women with low baseline mood . in addition , cognitive distraction during sexual activity is an established predictor of low sexual satisfaction ( dove and wiederman , 2000 ) . perhaps through these psychomechanistic pathways , androstadienone ( as well as estratetraenol ) were found to enhance sexual arousal induced by an erotic video ( bensafi et al . , 2004a ) . in addition , the application of breast and axillary pads from breastfeeding women over 3  months robustly increased sexual desire and sexual fantasies in recipient women ( spencer et al . , 2004 ) ; although it remains unclear whether putative pheromones from sweat or nursing secretions were responsible for this observation . activation of the autonomic nervous system produced sympathetic effects in women , cooling their hand palms and increasing their digit skin conductance ( jacob et al . moreover , salivary cortisol levels were higher after androstadienone exposure compared with vehicle ( wyart et al . , 2007 ) - perhaps reflecting a weak  cortisol awakening response  , well characterized following awakening in the morning ( elder et al . , 2013 ) . the brain areas activated by androstadienone are still unclear , with inconsistent results across studies ( jacob et al . preliminary data suggest that the brain processing of androstadienone and estratetraenol is different in heterosexual and lesbian women ( berglund et al . , 2006 ) . importantly , the positive effects of androstadienone on mood and skin conductance ( jacob et al . , 2001a ) as well as attention ( lundstrm and olsson , 2005 ) depended on the study attendant s gender , being present only with a male attendant . one might argue that , in freudian terms , the modulator effect of the pheromone was  transferred  onto the study attendant . in contemporary language , the pheromone was interpreted within a given ecological context . nonetheless , androstadienone also enhanced attention to emotional information in a more recent study which was performed largely without an attendant ( hummer and mcclintock , 2009 ) . in conclusion , the upper - lip application of androstadienone ( and to a lesser extent estratetraenol ) was associated with a positive mood and greater focus  in particular to capture emotional information  in women . however , these effects were dependent on the socioexperimental context . for example , in a double - blind experiment , recipient women who daily applied onto their upper lip a donor axillary pad extract , reduced their cycle - day difference with donor women from a mean of 8.3 days to a mean of 3.9 days over 3 cycles . in contrast , recipients who applied a blank solution had on average 6.2 cycle - days difference with these donor women at the start and 7.6 days after 3 cycles ( preti et al . , 1986 ) . the effect may be mediated by alterations in hypothalamic - pituitary regulation , since exposure to male axillary extracts shortened the lh - pulse interval by ( a modest ) 20% ( preti et al . , 2003 ) . yet strassmann ( 1999 ) has criticized some studies that purportedly show a role for pheromones in menstrual synchrony ( stern and mcclintock , 1998 ) . she notes that no anthropological evidence for synchrony could be obtained in african communities living close to one another . evolutionary speaking , amenorrhea was ( is ) the  gold standard  with most women in their reproductive years either pregnant or lactating in times of plenty , and in ( hypogonadal ) amenorrhea to save micronutrients in times of famine or war . menstrual synchrony is no priority when menses are few ( kung women in the kalahari only had a mean of 48 menstrual periods during their lifetime ) . in addition , a significant fraction of menses were ( are ) the result of early pregnancy failures ; and age > 35 years adds intra- and inter - individual variation in cycle length owing to ( non - synchronizable ) ovarian follicle depletion . in conclusion , any potential effects of pheromones on the menstrual cycle need further validation . at the present time , no firm conclusions can be drawn . adult humans can identify their own worn t - shirts out of 100 identical shirts worn by others ( lord and kaszprak , 1989 ) . family members can also identify t - shirts belonging to their partners , children and siblings ( porter and moore , 1981 ; wysocki and preti , 2004 ) . even breastfed infants can identify the axillary pad belonging to their mother  assessed by the duration of orientation of their heads toward the pad ( cernoch and porter , 1985 ) . in several species , mating is  disassortative  regarding the major histocompatibility complex ( mhc ) genotype , also known as the human leukocyte antigen ( hla a , b , c , dr , dq ) system in humans : i.e. , individuals prefer a mate with a dissimilar mhc - genotype , probably in an attempt to protect their offspring from homozygosity - induced illness . mhc - heterozygosity is more frequent than expected by chance . for dissimilar mate selection , the senses are critical : the visual sense for face and body scanning and attributing  attractiveness  to them , the auditory sense for vocal characteristics , and the olfactory ( and vno in non - humans ) sense for body odors . although the research is still ongoing , there is no evidence to date that facial preference is mhc - disassortative , in fact one study showed a preference for hla - similar faces ( reviewed in havlicek and roberts , 2009 ) . on the other hand , most although not all studies confirmed that body odor preference is indeed hla - disassortative ( havlicek and roberts , 2009 ) . if body odor scanning is important for mate selection , do the 16-androstenes  the most important pheromones identified at this time  play a mediating role ? saxton et al . ( 2008 ) used a real - world approach to examine the effect of androstadienone : speed - dating events . they set up 3 different experiments with 12 - 25 women and 19 - 25 men , and 3-minute interactions between them ; women scored the attractiveness of the potential date while being exposed to a cotton wool pad under their noses containing either androstadienone in 1% clove oil , 1% clove oil , or water . in two of the 3 experiments , the attractiveness ratings were better with androstedienone than water , and in 1 experiment better with androstedienone than clove oil only . in another study , there was a correlation between women s preference for masculine faces for a long - term relationship and their pleasantness rating of androstadienone ( but not androstenone ) ( cornwell et al . , 2004 ) . in conclusion , preliminary studies suggest that pheromones play a role in disassortative mate selection through the attribution of attractiveness . currently , the 16-androstenes , and in particular androstadienone is fairly well characterized as a pheromone for women . a positive mood and heightened focus are important for women s sexual response and sexual satisfaction . however , further study is required to validate these conclusions before any commercial claim of the  arousing  activity of androstadienone can be endorsed scientifically . preliminary evidence suggests that androstadienone also plays a role in mate selection , perhaps through the attribution of attractiveness to facial and bodily features of potential mates . as the pheromone field matures , the ideal study does not disclose the purpose of the study at all and does not involve skin application of compounds with masking odors , but introduces the experimental pheromone subliminally and with no additive to the participants ( in the room air ) while strongly controlling the psychosocial setting of the experiment .
pheromones are substances which are secreted to the outside by an individual and received by a second individual of the same species . 
 many examples exist in animals but their role in humans remains uncertain since adults have no functioning vomeronasal organ , which processes pheromone signals in animals . 
 yet pheromones can be detected by the olfactory system although humans under develop and underrate their smelling sense . 
 pheromones may be present in all bodily secretions but most attention has been geared toward axillary sweat which contains the odorous 16-androstenes . 
 one of these steroidal compounds , androstadienone , is present at much higher concentrations in male sweat and can be detected by women , albeit with wide variation in sensitivity . 
 upper - lip application of a pharmacological dose of androstadienonein women results in improved mood and heightened focus - particularly to capture emotional information . 
 a positive mood is known to facilitate women s sexual response , and increased focus improves sexual satisfaction . 
 indeed , some studies showed a beneficial effect of androstadienone on sexual desire and arousal . 
 however , these effects were dependent on the context of the experiment , for example , on the presence of a male attendant . 
 pheromones may also play a role in mate selection which is  disassortative  regarding the human leukocyte antigen ( hla)-genotype . 
 preliminary evidence suggests that exposure to androstadienone in women promotes attractiveness ratings of potential mates . in conclusion , some data indicate that 16-androstene pheromones , in particular androstadienone , play a beneficial role in women s mood , focus and sexual response , and perhaps also in mate selection .