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Evolutionary psychology is famous for having lots of stories that make sense but are hard to test. Psychiatry is famous for having mountains of experimental data but no idea what's going on. Maybe if you added them together, they might make one healthy scientific field? Enter Evolutionary Psychopathology: A Unified Approach by psychology professor Marco del Giudice. It starts by presenting the theory of "life history strategies". Then it uses the theory – along with a toolbox of evolutionary and genetic ideas – to shed new light on psychiatric conditions.

Some organisms have lots of low-effort offspring. Others have a few high-effort offspring. This was the basis of the old r/k selection theory. Although the details of that theory have come under challenge, the basic insight remains. A fish will lay 10,000 eggs, then go off and do something else. 9,990 will get eaten by sharks, but that still leaves enough for there to be plenty of fish in the sea. But an elephant will spend two years pregnant, three years nursing, and ten years doing at least some level of parenting, all to produce a single big, well-socialized, and high-prospect-of-life-success calf. These are two different ways of doing reproduction. In keeping with the usual evolutionary practice, del Giudice calls the fish strategy "fast" and the elephant strategy "slow".

To oversimplify: fast strategies (think "live fast, die young") are well-adapted for unpredictable dangerous environments. Each organism has a pretty good chance of randomly dying in some unavoidable way before adulthood; the species survives by sheer numbers. Fast organisms should grow up as quickly as possible in order to maximize the chance of reaching reproductive age before they unpredictably die. They should mate with anybody around, to maximize the chance of mating before they unpredictably die. They should ignore their offspring, since they expect most offspring to unpredictably die, and since they have too many to take care of anyway. They should be willing to take risks, since the downside (death without reproducing) is already their default expectation, and the upside (becoming one of the few individuals to give birth to the 10,000 offspring of the next generation) is high.

Slow strategies are well-adapted for safer environments, or predictable complex environments whose intricacies can be mastered with enough time and effort. Slow strategy animals may take a long time to grow up, since they need to achieve mastery before leaving their parents. They might be very picky maters, since they have all the time in the world to choose, will only have a few children each, and need to make sure each of those children has the best genes possible. They should work hard to raise their offspring, since each individual child represents a substantial part of the prospects of their genetic line. They should avoid risks, since the downside (death without reproducing) would be catastrophically worse than default, and the upside (giving birth to a few offspring of the next generation) is what they should expect anyway.