As I mentioned in the last post, I think a good deal of opposition, to evolutionary theory in general and evolutionary theory when directed towards psychology specifically, is due to a certain fear of moral exculpation, with other concerns about evidence or method being post-hoc rationalizations for that unease. For some, the human mind is somehow different, escaping either the need for an evolutionary analysis or the ability to be explained by one. There are those, like Rene Descartes, who think that the human mind - or at least parts of it - is not even a physical thing to begin with, but rather some immaterial essence.
With a haircut like that, it's probably a good idea to downplay physical features.
Today, I'll talk about this man's balls.
If Rene was anything like the average man alive today, his balls probably weighed between 40 to 50 grams, accounting for about 0.08% of his total - and very material - body's mass (Smith, 1984; Dixson, 2009). For the percentage of you who aren't either still giggling at the thought of balls (which accounts for roughly 100% of the men reading this) or questioning my sexuality, you might be curious why normal people would care about the size of balls in the first place.
To sate that curiosity of balls, there are many more balls to consider, but let's just stick to two groups: the balls of some of our close evolutionary relatives, the chimpanzee and and gorilla.
Warning: one of these pictures contains a graphic depiction of huge balls... wait, where are "warnings" supposed to go?
The chimp's balls weigh in at an impressive 120-150 grams, whereas the gorilla's come in at a combine 30ish. While the chimp may out-ball the gorilla by 4 or 5 times, that difference is actual an underestimate, as the gorillas are far larger in overall body size. When we adjust for the differences in body size, the gorilla's balls account for a mere 0.031% of their body weight, whereas the chimp's balls account for about 0.3% of their body weight. Pound for pound, chimps have about 10 times as much in their sack as gorillas.
Sure, the TruckNutz may look big, but once you take the mass of the vehicle into account....
So where does all this talk of balls leave us? It helps to know one last fact: the size of the testes, relative to the body, correlates to patterns of mating - not to their ability to kickass, as many men seem to think. When the sperm from more than one male are in contest to fertilize the same egg(s), we, in the creative names department, call it sperm competition. The gorillas, with their tiny ball-to-body size, face almost no sperm competition; they typically mate in a polygynous fashion, where one dominant male has uncontested sexual access to a harem of females. Chimps, on the other hand, live in multi-male/multi-female groups and, while the females are certainly not without preference, they mate in a far more promiscuous fashion.
So what about humans? We're certainly less ballsier than chimps - by about 400% - but definitely ballsier than gorillas - by about 300%, which tells us our species has probably faced some degree of sperm competition over our evolutionary history, milder than chimps but more intense than gorillas. Knowing these facts help guide us towards some potential conclusions about the human mind, bringing us nicely back to Descartes.
Rather than viewing the body and mind as two distinct pieces, the body can help inform us as to the psychology of the species; our bodies (and minds) are kind of like time capsules of evolutionary pressures. Without females historically mating with more than one male within her fertile window, or without males forcing copulation, there would be no potential for sperm competition. Of course, without females desiring to mate with more than one male and/or males desiring to mate irrespective of the female's wishes, no selection pressure would exist either.
Which reminds me how one of my professors - at the graduate level, no less - was trying to account for sex differences in behavior by simply appealing to body size differences between men and women, rather than differing psychologies. What he appeared to forget is that those body size differences require an explanation in the first place, and that explanation will ultimately returns to differences in psychology. Having the tools available without the will or knowledge to use them isn't much better than not having the tool at all, and vice versa.
(For those interested folk who like looking at naked organs, I'm told this is a comparison between a chimp's balls and brains: http://www.atomicnerds.com/wp-content/uploads/2010/08/Picture1-5.png)
Resources: Dixson, A.F. (2009). Sexual Selection and the Origins of Human Mating Systems. New York, NY: Oxford University Press
Smith, R.L. (1984) Sperm Competition and the Evolution of Animal Mating Systems, New York, NY: Academic Press.