Darth Vader had one thing going for him: a deep voice.
The ranks of George Clooney, Denzel Washington, Clint Eastwood, Don LaFontaine, and Barry White includes a common factor: A lower pitched voice—considered a positive masculine feature associated with with older, heavier, taller, hairier, and more attractive men (1). Studies have demonstrated a female preference for men with deeper voices as short-term partners (and preference seems to vary across the menstrual cycle, peaking during the height of fertility) (2,3). And elsewhere, research finds that North American men with lower-pitched voices report higher numbers for sexual partners in comparison to men with higher-pitched voices (4); and that Hazda men with lower-pitched voices have more living offspring (pitch is not an indicator of fecundity, but mate suitability) (4). Sexual selection has been proposed as a reason for deeper voices—the timbre and pitch suggest an attractive, fertile encounter. But a December PLoS paper reports that men with deeper, attractive voices have lower sperm quality than men with less attractive voices. Is there a evolutionary basis for voice preference?
There is certainly a link between testosterone and voice pitch: when testosterone levels begin to rise during puberty, it triggers changes in the larynx and in the vocal cords resulting in lower pitched voices. So deeper voices become associated with other manifestations (like facial hair) of testosterone, and consequently, perceived sexual fitness. Women (and likely men) consistently make positive judgments about masculinity based on voice pitch that include both physiological and behavioral traits. In addition to the characteristics noted above, men with lower pitched voices are perceived as being physically larger (taller, heavier) and are believed to be better fighters and providers (4).
These assessments aren't entirely made up. There is evidence that secondary sexual traits can predict health and fertility of a partner. Brilliant colors and showy displays have long been natural indicators of potential sexual fitness. For example, deer with bigger, more complex antlers also have larger testes and more motile sperm (5). Lower frequency sounds have been linked to larger body size across all primate species:
"The vocal tract is made up of hard tissue, its length being related to both
skull and skeletal size and the size of the tract determines the resonance frequencies of calls. The resonance frequencies, known as formant frequencies, are emphasized frequencies within vocalizations. [In] rhesus macaques, Macaca mulatta, the length of the vocal tract and the formant frequencies produced are both related to body size (6)."
Essentially, larger individuals have smaller differences between formant frequencies, which results in lower-pitched vocalizations. It is possible that at some point in our evolutionary history, vocal pitch may have been an important factor in mate determination, working in much the same way as other displays in the animal kingdom.
However, semen analysis reveals that men with deeper voices have lower scores on seven motility parameters (7)—even when the lifestyle and environmental factors are accounted for. While men with deeper voices may have more sexual partners, they seem less prepared to pass on their genes.* Researchers believe the lower sperm quality reflects a trade-off that comes with having to compete for mates:
"Animals have finite resources to partition amongst reproductive activities, and the theoretical models of sperm expenditure assume a basic trade-off between male investment in attracting mates and in gaining fertilizations. Recent studies of non-human animals are providing empirical evidence for this basic life-history trade-off. A number of studies have also reported short-term declines in semen quality associated with social dominance. In domestic fowl, Gallus gallus domesticus, and arctic charr, Salvelinus alpinus, for example, males becoming more dominant after a social challenge show a reduction in semen quality, while in cockroaches, Nauphoeta cinerea, both dominant and subordinate individuals suffer a reduction in ejaculate sperm counts resulting from the establishment of dominance hierarchies (4).
It appears then, that this secondary sexual trait which is linked to sexual maturity has become tied to social signs of maturity as well—taken to be economic and social stability. Which is a perception we find duplicated in popular culture:
"In movies and television larger men have deeper voices. This may reflect preconceptions, or may give rise to those preconceptions. Whichever is cause and which effect, it seems certain that larger men are expected to have deeper voices (8).
The broad-shouldered male hero has long stood as a symbol of status, meant to be respected and held to be an object of admiration. Male movie villains with deep voices are notable in that they appear powerful and inspire fear. Lest we fall into the trap of blaming everything on media, in the real world the perceived stability resulting from a long career and respect within a community may be tied to age and accomplishments, but seem to be viewed as general indicators of maturity, and as such are correlated with secondary sexual characteristics.
However, perhaps there's a shift occurring: Male heroes aren't always of the deep-voiced variety—and there has been a tendency to depict the deep-voiced, broad shouldered male hero as a bumbling idiot/arrogant fool/less intelligent companion (e.g., see such favorites as How To Train Your Dragon and Shrek (Prince Charming). In these instances, proof of ability outweighs assigned skills. That is, having a deeper voice doesn't necessarily grant one status ... though it might make for an delightful musical interlude.
*Clearly, not all men with deeper voices are infertile. They seem to do just fine generally. (Maybe those deeper voices help increase opportunities for them to pass on those genes—we just don't know. But we can talk about it.) In any event, it bears noting that lower sperm quality is NOT the direct result of having a deeper voice. That is, once your voice deepens, your sperm count and motility don't drop off. If that were the case, most men would be in serious trouble once their voices changed during puberty. It is interesting, however, to note that this perceived marker of sexual prowess (an extremely deep voice) may not actually reflect the whole picture. And may be quite telling about our own perceptions of masculinity. [Added 01/08/12 kd]
1. Collins. (2000): 778. | 2. Simmons (2011). | 3. Collins (2000) | 4. Simmons (2011): 4. | 5. Simmons (2011): 1 | 6. Collins (2000): 773. | 7. Average path velocity, straight line velocity, velocity along the sperm cells point-to-point track, lateral amplitude of sperm movement, frequency with which the sperm head crosses the average sperm path, the straightness of the sperm's path, and the linearity of the sperm's path. | 8. Collins (2000): 778.
Collins SA (2000). Men's voices and women's choices. Animal Behaviour, 60 (6), 773-780 PMID: 11124875
Feinberg, D., Jones, B., Little, A., Burt, D., & Perrett, D. (2005). Manipulations of fundamental and formant frequencies influence the attractiveness of human male voices Animal Behaviour, 69 (3), 561-568 DOI: 10.1016/j.anbehav.2004.06.012
Simmons, Leigh, Peters, Marianne, & Rhodes, Gillian (2011). Low Pitched Voices are Perceived as Masculine and Attractive but Do They Predict Semen Quality in Men? PLoS One, 6 (12), 1-6 : 10.1371/journal.pone.0029271