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ost conversations about why men cheat involve a lot of armchair psychology and not a lot of actual science. Psychologist Dr. Wendy Walsh has a different angle — and it starts with primate anatomy.
The short version: across primate species, there is a measurable correlation between the size of a male's scrotum relative to his body weight and how promiscuous that species tends to be. This isn't a joke. Anthropologists actually use this as a research tool.
Here's how it breaks down. Chimpanzees sit at one end of the spectrum — relatively small bodies, notably large gonads, and extremely promiscuous sexual behavior. Orangutans are the opposite — big hulking frames, small testes, and strongly monogamous. Where do humans land? Somewhere in the middle, which tracks with the fact that we have, as Walsh puts it, "the widest range of sexual behavior of any primate species." We're capable of everything from lifelong monogamy to enthusiastic promiscuity, and plenty of arrangements in between.
So does that mean you can clock a cheater by his anatomy? Walsh says early research does suggest a correlation — bigger relative to body size, higher likelihood of promiscuous behavior — and points to the fact that we already intuitively connect this idea to aggression and risk-taking in our everyday language. When we say someone has "big balls," we mean they're fearless, aggressive, high-testosterone. Those same traits, she notes, are exactly what you'd need to be a cheater.
Before anyone starts quietly doing comparative research, Walsh is clear that biology is not destiny. Two factors in particular show up consistently in research as buffers against cheating regardless of anatomy. The first is intelligence — not because smart men are more moral, but because they're better at running the numbers. Divorce is expensive. Single-parent households are hard on kids. A high-functioning cost-benefit analysis turns out to be a surprisingly effective check on biological impulse. The second is religion, which across nearly every tradition emphasizes family structure and fidelity, giving men an external moral framework that can override an internal biological one.
The broader point Walsh is making is actually one that applies well beyond the specific question of cheating: environment shapes behavior even when biology creates a strong pull in a particular direction. A genetic predisposition toward heart disease doesn't guarantee a heart attack if the lifestyle factors aren't there. A man can be born with whatever anatomy he was born with and still make choices that run counter to what the primate research might predict.
It's a useful framework for thinking about human sexuality generally — one that takes biology seriously without treating it as an excuse, and gives culture, intelligence, and individual choice real weight in the equation. Which is a more honest and interesting conversation than most of what gets written about why people cheat.
The full explainer is worth a watch. It's three minutes long and contains more genuinely useful information about human sexual behavior than most think pieces published in the last decade. Watch the video on Dr Walsh’s YouTube below 👇🏻
