If wheat really does make people better at mentalizing, and if wheat really does go hand in glove with Christianity for that reason, then consider how cultural evolution would work here. Christianity, being a powerful tool for big-scale cooperation, is “selected for” because big-scale cooperation wins against small-scale cooperation, and thus wheat, being something that makes it easier for people to believe in God, be Christian, and thus fit into that big-scale cooperation system, is “selected for.” The groups that didn’t emphasize wheat in their diet as much wouldn’t have had as much conviction about, say, the Christian “contingent afterlife”—they wouldn’t have been as “schizophrenically sure” of that unseeable, unknowable “fact”—which would have made them less likely to follow the big-scale-cooperation-related Christian rules.

It’s important to take into account, though, the dosage. It’s not only possible for the people in a group to change how much wheat they eat, which would change the dosage. It’s also possible to change the wheat itself. That’s where artificial selection comes in. As long as better mentalizing (for the sake of Christianity) was being (naturally) “selected for” on the group level, the groups that stumbled onto wheat artificial-selection patterns that changed wheat in order to make it even better at making people better at mentalizing would have been (naturally) “selected for.” Artificial selection is powerful—just look at dogs, for instance—and thus it’s clear that Christian civilization’s wheat may well be very different than whatever wheat was originally.

The argument generalizes: A group’s diet is likely to fit hand in glove with the group’s culture (in how the diet affects psychology), especially if the group is successful. Our hunter-gatherer ancestors (at least when they had the luxury of plenty of food to go around) were able to choose whether or not to eat, say, meat, but they weren’t able to change the meat iself. In civilization, though, because of the agricultural revolution, there’s artificial selection. Groups are able to artificially select domesticated animals, and the artificial-selection patterns are themselves (naturally) “selected for.” Same for domesticated plants.

(Besides wheat, dairy is another example of a strongly psychoactive food, having opioids too, and civilization has strongly artificially selected cows. That artificial selection probably has not only economic but also psychological import.)

By analogy, consider that whether it’s light or dark, hot or cold, affects psychology. Our hunter-gatherer ancestors had less control over that, but in civilization, we have more control. Cafes, for instance, are dark and cold (relatively speaking), because that’s conducive to studiousness. In the same way, food affects psychology, especially strongly psychoactive foods like wheat, rice, and dairy, and civilization has taken control.