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Our co-blogger here at Psychology Today, evolutionary psychologist Satoshi Kanazawa, recently posted an explanation of how we know that "women have always been mildly promiscuous throughout human evolutionary history."
Problem is, we don't.
To be fair, the argument Kanazawa presents has been around for a long time and is pretty straightforward. Among mammals, those with promiscuous mating systems tend to evolve larger testicles, relative to their body size, than those with more limited mating behavior. Fine.
But then Kanazawa (and many other evolutionary psychologists) take a bold step out on to very thin ice when he applies this simple observation to human evolution, writing,
"The evidence of women’s promiscuity throughout evolutionary history is in the relative size of men’s testicles. Men would not have such large testicles and produce so many sperm per ejaculate had women not been so promiscuous. But then, their testicles would have been much larger and they would have produced even more sperm per ejaculate had women been more promiscuous."
This only makes sense if we can be certain that human testicular volume is extremely static -- not changing over the tens or hundreds of thousands of years that make up "evolutionary history." And this is simply not the case.
As it turns out, testicular volume is extremely responsive to environmental factors ranging from exogenous estrogens, toxins in the air and water, and growth hormones used in the food supply, even the dictates of fashion (think: tight jeans). Men's testicles shrink with age, and there are marked differences in the average testicular ratio of Africans, Europeans and Asians -- which calls into question the notion that testicular ratio is some universal figure based upon extended evolutionary processes. At least one primate, the capuchin monkey, experiences radical changes in his testicular size seasonally!
So as far as we know, there is no reason to believe that human testicular ratios offer reliable evidence of anything other than current conditions. If the size of human testicles changes with environmental (and possibly cultural) factors, then certainly their current state cannot be used to support tenuous conclusions about conditions 100,000 years ago.
