Human voices are sexually dimorphic in at least three ways: males have lower fundamental frequency, lower and more closely spaced formant structure, and less fundamental frequency variation. The overarching goal of the present research is to understand the ultimate and ontogenetic origins of human vocal dimorphism using the theory of sexual selection. Outstanding questions can be organized into three general lines of inquiry: 1) Have male voices been subject to both intra- and intersexual selection? Or has one process dominated? 2) What does a masculine voice signal? That is, why would potential mates and competitors attend to vocal characteristics? 3) What constraints ensure that vocal signals remain honest, or at least honest enough to warrant attention? Part A (Articles 1 and 2) targets question 1 by examining reactions to masculine voice features among college-aged men and women.

Part B (Articles 3 through 6) primarily targets questions 2 and 3 by studying individual differences in vocal maturation among Bolivian adolescents. Results include several important findings. US college-age men's mating success (Article 2) was better predicted by a strong correlate of vocal dominance (F 0 variation; Article 1) than by a strong correlate of vocal attractiveness (mean F0; Article 1), suggesting that intrasexual selection may have been an important selection pressure on the evolution of men's vocal traits, and may be more important in that regard than female choice. Listeners may attend to vocal frequencies because they signal honest, non-redundant information about male strength that cannot be ascertained from visual inspection of body size (i.e., height or adiposity; Article 4).

Tsimane males experience later voice change and have higher adult voices compared with North Americans (Article 3), suggesting that a higher-pitched voice reveals a developmental history of energetic constraints. Not only is testosterone strongly related to development of fundamental and formant frequencies, but it mediates the relationship between energetic condition and vocal characteristics (Article 5). These findings provide evidence for a costly signaling model of masculine voices; males in better condition have greater resources to invest in testosterone-dependent traits like the voice.