Human Courtship Adaptations:

Nonhuman vertebrate species express biological adaptations that facilitate courtship and mate competition. In males of most sexually reproducing species, for instance, individuals exhibit a type of "mating response" when they encounter potential mates: species-specific courtship behaviors, combined with rapid increases in testosterone and corticosterone. These responses, furthermore, are regulated by structures within a specific limbic-hypothalamic circuit, such as the medial preoptic area. The phylogenetic conservation of this circuit in turn raises the possibility that humans express similar (homologous) mechanisms. Our lab has an ongoing research program testing whether in fact human males also exhibit a mating response similar to that seen in nonhuman vertebrates. Four published studies have now provided evidence that men exhibit reactive testosterone increases after brief social interactions with young women, for instance, and three studies have provided evidence for reactive increases in cortisol as well:

Roney, J. R., Mahler, S. V., & Maestripieri, D. (2003). Behavioral and hormonal responses of men to brief interactions with women. Evolution and Human Behavior, 24, 365-375.
Roney, J. R., Lukaszewski, A. W., & Simmons, Z. L. (2007). Rapid endocrine responses of young men to social interactions with young women. Hormones and Behavior, 52, 326-333.
Roney, J. R., Simmons, Z. L., & Lukaszewski, A. W. (2010). Androgen receptor gene sequence and basal cortisol concentrations predict men's hormonal responses to potential mates. Proceedings of the Royal Society of London B, 277, 57-63.

In addition to investigating hormonal responses to potential mates, an early study also demonstrated that mere exposure to young women can nonconsciously prime psychological changes in men that appear designed to promote more attractive self-presentations to potential mates:

Roney, J. R. (2003). Effects of visual exposure to the opposite sex: Cognitive aspects of mate attraction in human males. Personality and Social Psychology Bulletin, 29, 393-404.

Our ongoing investigations are designed to extend this research in various ways. A current set of experiments being directed by Zach Simmons for his dissertation are investigating the possible functions of rapid, transient increases in testosterone. A major function of testosterone appears to be the regulation of energy investment into either mating or survival effort (broadly construed), with higher testosterone generally associated with greater relative investment in mating effort, and greater demands on survival effort associated with decreases in testosterone. In theory, for example, testosterone should decrease during immune activation in order to allow more energy to be allocated to the immune response, and one paper from our lab contributed evidence for this effect in humans:

Simmons, Z. L., & Roney, J. R. (2009). Androgens and energy allocation: Quasi-experimental evidence for effects of influenza vaccination on men's testosterone. American Journal of Human Biology, 21, 133-135.

Transient increases in testosterone seen after exposure to potential mates may provide a signal regarding the immediate importance of investment in mating effort that serves to calibrate psychological mechanisms toward mate competition in the relative short-run, much as higher basal production of testosterone may allocate energy resources into mating effort (e.g., into the construction of muscle mass) over longer time scales. This psychological calibration toward mate competition may include outcomes such as lower risk-aversion, greater competitiveness, and greater willingness to approach potential mates, all of which are being tested in the ongoing research.


The Role of Ovarian Hormones in Women's Mating Psychology:

An intriguing recent body of research has provided evidence that aspects of women's mating psychology undergo changes associated with phases of the menstrual cycle. Preferences for more masculine features in men  (e.g., faces, voices, bodies), for instance, are stronger when women are tested near ovulation than when tested during other phases of the cycle. The most prominent proposed explanation for these cycle phase shifts is that they are part of a specially designed "mixed" mating strategy in which women prefer to form pair-bonds with high investing men who are good fathers but also opportunistically commit infidelities near ovulation in order to obtain higher quality genes than may be available from their long-term partners. On this account, masculine features index aspects of heritable fitness, and women exhibit relatively stronger preferences for these traits specifically during fertile days of the cycle, since only on these days can the genetic benefits of infidelities outweigh the risks associated with extra-pair sex.

Our lab is testing an alternative functional explanation for cycle phase shifts in mate preferences. This explanation proposes that these shifts are essentially the products of mechanisms that allocate attention and motivation differentially depending on the most important adaptive problems currently being faced by an individual. During long stretches of infertility associated with events like lactation or energy shortage, it may have been functional for ancestral women to down-regulate attention to men's sexual attractiveness in order to focus on more pressing current problems, but to up-regulate such attention in the more rare cycles in which fertility returned. If, furthermore, a hormone like estrogen (known to be higher in more fertile cycles) is the physiological signal regulating these shifts, then a mechanism designed primarily to shift attention and motivation between different cycles might also produce small within-cycle shifts as a by-product of its design, since estrogen peaks near ovulation within cycles. Further discussion of this theory can be found in:

Roney, J. R. (2009). The role of sex hormones in the initiation of human mating relationships. In P. T. Ellison & P. B. Gray (Eds.), The endocrinology of social relationships (pp. 246-269). Cambridge, MA: Harvard University Press.

Empirical tests of the physiological signals that regulate cycle phase shifts in attractiveness judgments can help provide evidence regarding the alternative functional theories. Estrogen, for example, typically exhibits a within-cycle peak near ovulation, but also often a secondary peak in the luteal phase when conception is not possible. If preferences for masculinized traits track estrogen fluctuations not only near ovulation but also into the luteal phase, this pattern would be consistent with a between-cycle mechanism that adjusts psychology according to cycle fertility but less consistent with a specialized infidelity-promoting mechanism that if well-designed should tightly couple preference shifts to days of the cycle when conception is possible. Three studies from our lab have provided evidence that preferences for some androgen-dependent traits do in fact track estrogen fluctuations across the cycle, including past ovulation and into the luteal phase:

Roney, J. R., & Simmons, Z. L. (2008). Women's estradiol predicts preference for facial cues of men's testosterone. Hormones and Behavior, 53, 14-19.
Lukaszewski, A.W., & Roney, J. R. (2009). Estimated hormones predict women's mate preferences for dominant personality traits. Personality and Individual Differences, 47, 191-196.
Roney, J. R., Simmons, Z. L., & Gray, P. B. (2011). Changes in estradiol predict within-women shifts in attraction to facial cues of men's testosterone. Psychoneuroendocrinology, 36, 742-749.

One limitation of the above studies is that they involved only 1-2 data points per woman, and much more detailed information could in principle be obtained by following the same women across more days of their respective cycles. Our ongoing research is doing just that: daily saliva samples, daily diaries, and multiple stimulus rating lab sessions per woman subject are being collected and analyzed in a current study.


The Functional Origins of Personality and Individual Differences:

Why do people differ in their personalities? In other traits? A prominent idea in the current literature is that genetic differences explain much of the relevant variation: people differ in extraversion, for instance, because some people have genes for extraversion and some genes for introversion. An alternative idea, however, is that personality traits may be adjusted in functional ways depending on feedback received over the course of development. As part of his dissertation, Aaron Lukaszewski applied this "facultative calibration" hypothesis to the explanation of individual differences in extraversion. Based on the expectation that extraverted behavioral strategies would likely have had higher payoffs for stronger and more physically attractive individuals across most of human history, this research tested and confirmed the hypotheses that strength and attractiveness are positively correlated with self-reported levels of extraversion. The relevant studies have recently been published:

Lukaszewski, A. W., & Roney, J. R. (2011). The origins of extraversion: Joint effects of facultative calibration and genetic polymorphism. Personality and Social Psychology Bulletin, 37, 409-421.

The above paper also found that the effects of strength and attractiveness in men were statistically independent of variance in extraversion explained by an androgen receptor gene polymorphism, despite the fact that variation in this gene was significantly correlated with both strength and extraversion. That result argues for the simultaneous effects of facultative calibration and genetic polymorphism, suggesting the importance of both in explaining the origins of personality.

The androgen receptor gene is of broader interest to our lab because of its potential to explain individual differences associated with mating psychology. Shorter numbers of CAG repeats in this gene are essentially associated with more active androgen receptors. Since androgen receptors are expressed widely throughout the brain and body, adjusting their activity via this one gene can potentially alter the degree of androgenicity throughout the entire organism. Because androgens appear to regulate relative investment in mating (vs. survival) effort, shorter CAG repeat lengths in this gene may predict trait-like individual differences in variables associated with mating effort. Consistent with this, one paper from our lab demonstrated that men with shorter repeat lengths exhibited larger testosterone responses to interactions with potential mates:

Roney, J. R., Simmons, Z. L., & Lukaszewski, A. W. (2010). Androgen receptor gene sequence and basal cortisol concentrations predict men's hormonal responses to potential mates. Proceedings of the Royal Society of London B, 277, 57-63.

A follow-up paper has reported significant associations between CAG repeat length and variables associated with intrasexual competitiveness in men (physical strength and  self-reported levels of dominance and prestige):

Simmons, Z. L., & Roney, J. R. (2011). Variation in CAG repeat length of the androgen receptor gene predicts variables associated with intrasexual competitiveness in human males. Hormones and Behavior, 60, 306-312.

Miscellaneous Research Projects:

A few other lines of research are also ongoing in our lab, which we hope to expand upon in the near future. First, we are interested in the information that observers are able to obtain from human faces. Previous research has shown that women exhibit stronger preferences for more masculine male faces when the women are tested near ovulation than when tested in other regions of the cycle. Masculinity in these studies has been interpreted as an index of both genetic and paternal quality: a positive indicator of genetic quality via its presumed association with higher testosterone, and a negative predictor of paternal quality based on studies showing that observers rate masculinized faces lower on perceived prosocial traits. In an initial study from our lab, we sought to test whether female observers were in fact accurate in their subjective impressions of male faces. We found that women's ratings of men's facial masculinity were positively correlated with the testosterone concentrations of the men depicted in the face photographs. Likewise, women's ratings of how much the men like children (from facial photos alone) were significantly and positively correlated with the men's scores on an interest in infants test. These findings are published in:

Roney, J. R., Hanson, K. N., Durante, K. M., & Maestripieri, D. (2006). Reading men's faces: women's mate attractiveness judgments track men's testosterone and interest in infants. Proceedings of the Royal Society of London B, 273, 2169-2175.

A number of investigations are ongoing as follow-ups to this paper, including cross-cultural investigations of rating accuracy, and studies attempting to identify the specific perceptual cues that may best index endocrine and attitudinal variables.

A second line of research is investigating new approaches to the study of human mate preferences. The mate preference literature has been largely dominated by self-report methodologies in which subjects simply rate or rank lists of traits for their importance in a potential mate. A common finding in this literature is that people prefer kindness and trustworthiness above all other traits in a possible partner, and generally rate traits like dominance as much lower in importance. In one study from our lab, though, we questioned whether these preferences depend on the targets of a partner's behavioral acts: that is, do people prefer that a partner be extremely kind specifically toward themselves, or toward other people in general? The results demonstrated that subjects exhibit preferences for extremely high levels of kindness and trustworthiness only when considering behaviors directed toward themselves or their friends/family, and shift to lower preferences for these traits (and higher preference for dominance) when considering a partner's behavior directed toward other classes of individuals. These findings are published in:

Lukaszewski, A. W., & Roney, J. R. (2010). Kind toward whom? Mate preferences for personality traits are target specific. Evolution and Human Behavior, 31, 29-38.

Finally, other ongoing studies are attempting to develop new methods of assessing mate preferences that do not rely entirely on self-report and its associated problems with respect to whether subjects have accurate, consciously accessible insight into the traits that generate feelings of attraction toward others. One approach involves policy-capturing methodologies that use ratings of perceptual stimuli in lieu of ratings of trait importance, for example, and we are currently in the process of preparing paper submissions based on the results of such studies.