My research integrates knowledge from multiple disciplines and employs a multi-method approach incorporating tools such as digital image and video manipulation, hormonal assays, and genotyping to answer previously unexplored questions about two distinct domains of human psychology: 1) standards of attractiveness and 2) individual differences in personality.

Research Program I: Previously Undiscovered Standards of Attractiveness

     A dominant view in the social sciences has been that human standards of beauty are arbitrary and entirely dependent on sociocultural processes. This view contrasts with a vast body of evidence from non-human species that an organism’s attractiveness as a mate is linked to traits that help solve survival- and reproduction-related challenges. In my research, I empirically test the overarching hypothesis that human standards of attractiveness reflect the output of psychological adaptations to detect survival- and reproduction-relevant traits.

     I consider survival- and reproduction-related challenges faced by ancestral humans, and then derive and test novel hypotheses about how these challenges would have shaped humans’ psychology of attraction. Convergent results from my published research (e.g., Lewis, Russell, Al- Shawaf, & Buss, 2015) provide evidence of previously unknown standards of attractiveness, and reveal a tight fit between a priori hypotheses based on evolutionary selection pressures and modern standards of attractiveness. My most recent research has tested hypotheses based on an adaptive problem uniquely faced by ancestral hominin females: a forward-shifted center of mass during pregnancy. The human female spine (but not the male spine) possesses evolved morphology to deal with this adaptive challenge: wedging in the third-to-last lumbar vertebra. Among ancestral women, an angle of vertebral wedging that minimized the net fitness threats posed by insufficient lumbar curvature (hypolordosis) and excessive lumbar curvature (hyperlordosis) would have been biomechanically optimal. The fitness advantages to women who possessed this degree of vertebral wedging would have, in turn, created selection pressures on men to prefer such women as mates. Based on this reasoning, I hypothesized that men possess an evolved mate preference for women exhibiting cues to this theoretically optimal degree of lumbar wedging.      

     Multiple independent studies employing distinct methods have provided robust, convergent evidence for this previously unknown standard of attractiveness. Precisely as predicted, women’s attractiveness peaks at a biomechanically optimal angle of lumbar curvature. In comparison, women prefer comparatively smaller angles of lumbar curvature in men, consistent with the notion that the uniquely female challenge of a bipedal fetal load favored a higher degree of lumbar curvature among women than men (Lewis, Al-Shawaf, & Russell, manuscript invited for resubmission). Subsequent studies have also ruled out alternative explanations such as the proposal that the observed preference can be attributed to a preference for larger buttocks. It cannot. Men consistently prefer women whose curvature reflects vertebral wedging, not buttock mass (Lewis et al., 2015a). Collectively, these findings reveal a previously undiscovered standard of attractiveness that fits seamlessly within a broader evolutionary framework.

     My research in this domain continues. Together with my team of four graduate students and four undergraduate thesis students, I am studying other morphological features of the human body that exhibit clear links to reproductive success—but whose influence on attractiveness has yet to be investigated. I am also examining how these standards of attractiveness influence women’s 1) self- perceptions; 2) perceptions of, attitudes towards, and affective responses to other women, including envy and jealousy; and 3) behavioral tactics to display, accentuate, or enhance these features on their own bodies. I am also studying how these female behavioral tactics influence men’s attraction to women. This multi-faceted research program highlights the core theme of my research: to inform research on contemporary psychological processes through evolutionary models that yield new, testable hypotheses about how humans’ psychological mechanisms operate in modern sociocultural contexts.

Research Program II: The Biological and Social Origins of Personality

     At present, the field of personality psychology offers a wealth of robust empirical research and has made major progress toward trait taxonomy. Nonetheless, the proximate and distal causal processes responsible for the development of personality traits – including those that are risk factors for impaired health and wellbeing – are not fully understood. My research objective is to apply a new interdisciplinary paradigm to increase our understanding of the evolutionary, genetic, endocrinological, morphological, and social etiology of these traits. My research bridges four important disciplines: social psychology with its emphasis on personal relationships, personality psychology with its focus on stable individual differences, behavior genetics with its focus on heritable and environmental causal influences, and evolutionary psychology with its focus on humans’ evolved psychological mechanisms. By bringing these branches of knowledge together, my research (Lewis, 2015; Lewis, Al-Shawaf, & Yilmaz, 2015) offers a novel framework for predicting the specific conditions and social environments that evoke the development of distinct personality traits.

     My research reveals previously unknown biological and social causes of individual differences in personality (Lewis, Al-Shawaf, Janiak, & Akunebu, manuscript submitted for publication). I explore how individuals’ genes influence their social environments, and how this link between genes and social environment mediates the relationship between genetics and personality. For example, I test the hypothesis that evolved psychological mechanisms up-regulate neuroticism as a functional response to exclusion from social relationships. Specifically, I propose that individuals who possess fewer phenotypic indicators of genetic quality are treated as less desirable social partners and disproportionately experience social exclusion. Consequently, they exhibit elevated neuroticism, which guides their attention toward negative potential social outcomes and cognitively and affectively mobilizes them to protect their limited social opportunities.

     To provide a more comprehensive understanding of the causal pathways between genetics and personality, I employ methods and analytical tools from multiple disciplines. For example, I conduct fragment analysis at the androgen receptor gene, a polymorphic locus associated with the development of morphological characteristics known to be desired in social relationship partners. Complementing these genetic methods, I assay salivary testosterone, a hormone hypothesized to be an endocrinological marker of underlying genetic quality. I collect kinetic indices of physical health and strength using a hydraulic dynamometer; assess morphological indicators of mutation load with medical grade calipers and NIH image analysis software; and administer psychometrically validated instruments for measuring personality and other psychological variables.

     Results from independent studies conducted in multiple countries (e.g., Lewis et al., 2015b) provide empirical support for this novel framework for investigating the etiology of personality differences. This includes three distinct sub-studies conducted with a large sample of couples in long-term relationships (Lewis, Al-Shawaf, Li, & Buss, manuscript in preparation). In the first sub-study, I demonstrate that individuals’ desirability as a relationship partner predicts their experience of social exclusion, which in turn predicts their neuroticism. In the second, I show that men’s sexual jealousy and neuroticism are functionally calibrated according to their desirability as a mate and consequent threat of being excluded from mating relationships. As hypothesized, men’s mate value predicts their anxiety about their partners’ fidelity as well as their own frequency of mate guarding behavior. In the third, I demonstrate that exposing men to scenarios describing their mates’ certain fidelity, uncertain fidelity, or certain infidelity leads to individual differences in manifest neuroticism that directly correspond to the degree of relationship threat. Critical to the support of the main hypothesis, men’s desirability as a relationship partner systematically predicts their neuroticism when they are uncertain of their mates’ fidelity. These results suggest that evolved psychological mechanisms adjust individuals’ neuroticism levels as a functional response to relationship exclusion.

     More broadly, my published and ongoing research on personality – which spans multiple individual difference dimensions, from the Big 5 to adult romantic attachment style; employs both correlational and experimental designs; and is based on samples from multiple countries with different languages and cultures (e.g., see Lewis, Al-Shawaf, & Yilmaz, 2015) – suggests that investigating individual differences through this new interdisciplinary lens has the potential to illuminate previously unknown causal pathways to personality traits, including those linked to jeopardized mental health and impaired social wellbeing.