There is an issue currently being discussed on the Internet about the relationship between human psychology and human biodiversity. For example, take this post by Larry Arnhart where he wonders whether HBD threatens the “psychic unity of mankind”:

This suggests that the evidence of evolutionary trends towards declining violence that Steven Pinker has surveyed could be evidence not just of cultural evolution but also biological evolution. Remarkably, however, Pinker refuses to accept this conclusion because it contradicts the “standard assumption in evolutionary psychology” that human nature has not changed over the past 10,000 years and that this is supported by the “psychic unity of humankind” (612-13). This is the fundamental assumption of Leda Cosmides and John Tooby, the founders of evolutionary psychology.

I want to argue here that there is no conflict between HBD and the psychic unity of mankind, if this phrase is understood correctly. Specifically, mankind shares a biofunctional psychological architecture, despite differences that may occur due to local selection pressures.

We are by now familiar with evolutionary psychology when it comes to understanding certain things like altruism or sexual attraction. But there is another branch of evolutionary psychology that has received less public attention: biofunctional psychology. Biofunctional psychology looks to understand psychological states—beliefs, desires, intentions, feelings—the way a biologist looks at hearts, livers, and kidneys. That is, it looks to understand what it is these mental processes do (or better yet, what their ancestors did) that has proven to be evolutionarily advantageous. For example, hearts pump blood. That is what they do which benefits the organism, that is their function. Hearts also make a lub-dub sound, freeze when placed in liquid nitrogen, have a certain mass, shape, density, and so on, but it is by pumping blood that they benefit the organism. More specifically, of all the things hearts do, pumping blood is what our ancestors’ hearts did that benefited them and was selected for by natural selection.

The same approach can be given to the understanding of psychological states. For example, take hunger. What does the subjective feeling of hunger do for the organism that benefits it? The answer is that the function of feelings is to get the organism to perform a certain behavior–food procurement in this case. Other psychological processes can be given a likewise functional understanding. The function of beliefs, for example, is to be combined with other true beliefs in order to form new true beliefs in the process of inference, and ultimately to be invoked by desires in guiding them in successful actions. The function of desires is to produce the conditions of their own fulfillment. The function of emotions such as fear is to produce certain behavior; to seek safety in this case. Notice that the function of all psychological states is ultimately to contribute to successful behavior; beliefs are supposed to be true because it is by being true and representing the world in an accurate way that they may invoked by desires as useful guides for behavior. Those interested in biofunctional psychology should read Millikan’s White Queen Psychology and Other Essays for Alice.

In this sense the psychic unity of mankind is true in that the functional psychic architecture of humanity is universal. I don’t see any reason to believe that people differ from place to place in this respect; it is not as if there are some people who do not produce beliefs about the environment, combine those beliefs in inference, invoke those beliefs in pursuing desires, or produce characteristic behaviors as a result of subjective experience of fear, hunger, thirst, anger, and so on. There do not appear to be any human populations on earth lacking in any of these respects.

However, I find it plausible that people might differ in another way, or that natural selection might have selected for different psychological traits. Take the subjective feeling of hunger. What is it designed to do? Just as the function of sexual attraction is to get the organism to pursue sex, and thirst is to get the organism to get water, and fear is to motivate the organism to seek safety, the function of hunger is to get the organism to procure food. The functional way this occurs is for the feeling to produce an intention, and for this to produce a behavior that corresponds to the intention: hunger produces the intention to go to the kitchen which produces going-to-the-kitchen behavior. And many times this is all there is to the story. But of course we do not always act directly on our feelings because reasons may be inserted in the pathway between emotion and behavior: if we are hungry we may not go to the kitchen if we have a reason not to; maybe we are in the middle of some work that we want to get finished first.

However, reasons may be eventually overcome if emotions are strong enough. If my hunger gets strong enough it will overwhelm whatever reasons I have for not eating and will produce the appropriate food-getting behavior. I find it plausible that natural selection could work on the intensity of feelings, especially for anger. What behavior is anger designed to get the organism to do? My contention is that anger is designed to get the organism to produce violence towards the object of the anger. And just as we might not act on our hunger if we have reasons not to, we can learn to not act on our anger. But just as hunger has a threshold where we will ignore our reasons and act on the hunger, people are liable to lash out if they become intensely angry, or if the reasons for not acting on it (fear of punishment, for example) are removed.

If everything in the biological world in unequally distributed, we should expect people to vary in the intensity of their emotions. In this way the psychic unity of mankind is true if understood architecturally, but psychological HBD can still be true. There are probably people who feel different intensity of anger, or sexual desire, or fear. Women almost undoubtedly have a lower threshold for crying. (Bonus question: emotional displays are designed to get the perceiver of the display to perform a certain behavior. So what behavior is women’s crying designed to produce in its audience? And why would nature design women to have a lower threshold for crying?)

Cochran and Harpending claim that once people settled down in agricultural lifestyles, and hierarchies evolved, the rulers bred their subjects for passivity (probably not consciously, just by punishing those who were violent with death or imprisonment). If so, it probably worked by decreasing the intensity of anger, or raising the threshold where anger leads to violence. Just as those Russian fox breeders selected for those foxes which had a high threshold for fear of humans, it seems plausible to me that something similar could have happened in humans.
One interesting point is that the threshold for acting on emotions can be altered with experience. I know that when I was younger I had a very short temper. I had a couple of occasions where I scared myself by allowing my anger to control me and I had to work hard to get it under control. I think that probably, every human can similarly learn to control their emotions, although it might actually be harder for natural hot-heads who either have stronger intensity of emotions, or lower thresholds where they lose control.