So, it seems people have some pretty strong feelings about penises. Reactions to my last post—“Secrets of the Phallus: Why Does the Penis Look Like That?”— ranged from the incredulous (are you seriously suggesting that chimpanzees aren’t promiscuous?—“tomrees”), to the imaginative (penises! they're so cute, you just want to pinch their cheeks and give them cookies—“montavilla”), to the rather irritable (stupid, biased thinking again from an ‘evolutionary psychologist’—“hcollins2009”).

For some reason, when it comes to asking whether human beings have evolved some specialized trait over the past several million years I’ve found that people tend to get weirdly worked up about it. I once had the most unpleasant conversation with a very unlikable ornithologist while dining at a Japanese steakhouse in Binghamton. I think I was a hairsbreadth away from being strangled by this scientist who took considerable offense to my suggestion that magpie intelligence isn’t as humanlike as it may appear. But the subject of human penis evolution appears to have touched a special nerve.

Therefore, I decided to speak with evolutionary psychologist Gordon Gallup of the State University of New York at Albany directly, whose controversial “semen displacement theory,” after all, was the one that struck up such a fierce (and I must say, entertaining) brouhaha regarding the adaptive functioning of this enigmatic organ. I explained to him that there appeared to be a bit of confusion in the reading audience concerning some of the central evolutionary tenets of his position, and perhaps he might offer us a few more details regarding the theory to lay any recurring misunderstandings to rest.

In looking over the varied responses to the earlier post here at Scientific American and elsewhere on the Internet, Gordon and I noticed several conceptually flawed themes cropping up in people’s interpretations of his argument. Since it would be impossible for him to address every rejoinder to his semen displacement theory (and, frankly, some of them were so bizarre that I couldn’t make much sense of them anyway), I’ve translated a handful of these “core” questions below.

READERS: The latex genitalia study wasn't terribly convincing because the models were circumcised, and in real life the foreskin would interfere with the semen-displacing functions of the coronal ridge. So, does the foreskin pose a problem for the semen displacement theory?

GALLUP: The length of the foreskin is one of the most variable features of the human penis. When most uncircumcised males achieve an erection it pulls the foreskin back over the glans and back down the shaft of the penis, enabling the coronal ridge to do its business and scoop rival males’ semen away from the woman’s cervix. Because circumcision reduces the diameter of the shaft immediately behind the glans and accentuates the coronal ridge, we’ve speculated that the practice of circumcision may have unwittingly modified the penis in ways that enable it to function as a more effective semen displacement device. Armchair speculation? No. The idea could be tested by comparing the incidence of non-paternity between circumcised and intact males. My prediction would be that circumcised males ought to experience a lower incidence of being cuckolded.

READERS: So why did human penises evolve to have foreskin at all then?

GALLUP: Evolution does not occur by design. The best way to think about most adaptations is in terms of cost/benefit ratios. I suspect that the foreskin provided protection of the glans and what you see is the result of a statistical compromise of sorts.

READERS: If the penis really evolved to displace semen, then why wouldn’t other promiscuous primate species, namely chimpanzees, have evolved similarly-designed penises with the coronal ridge?

GALLUP: Again, evolution doesn’t occur by design. It occurs by selection, and the raw material for such selection consists of nothing more than random genetic accidents (mutations). Embedded in the evolutionary history of human genital design were some penis shape mutations, not present in other species, that led to a device that could be used to compete with other males for paternity. Other promiscuous primates such as chimpanzees have solved the problem through sperm competition. Male chimpanzees have testicles that are three times the size of humans and differences in sperm count are on the same order of magnitude. Chimpanzees compete among one another for paternity by leaving the largest and most potent volume of semen in the female reproductive tract. When it comes to selection based on genetic accidents, there are a number of ways to skin the adaptive cat.

Bering here. I should say in closing that those who’ve been intellectually conditioned to recoil in disagreement at the mere mention of evolutionary explanations of human behavior are likely to scream “Just-So Story!” no matter how convincing the argument and well-supported it is by empirical evidence. Some evolutionary theories indeed leave a lot to be desired. But, in the present case, the semen displacement theory just so happens to make a lot of adaptationist sense—and I suspect that once you know what you’re dealing with down there, you’ll probably never look at a penis quite the same way again.

In this column presented by Scientific American Mind magazine, research psychologist Jesse Bering of Queen's University Belfast ponders some of the more obscure aspects of everyday human behavior. Ever wonder why yawning is contagious, why we point with our index fingers instead of our thumbs or whether being breastfed as an infant influences your sexual preferences as an adult? Get a closer look at the latest data as “Bering in Mind” tackles these and other quirky questions about human nature. Sign up for the RSS feed or friend Dr. Bering on Facebook and never miss an installment again.