Cliff-edged fitness functions

A very interesting 2004 BBS article by Jonathan Burns asked why genes that cause schizophrenia persist even though they decrease reproduction.  That inspired me to spend a month full-time thinking about what kind of explanation could possibly work. Finally, I recognized that traits with an asymmetric fitness function are special; they give greater and greater advantages up to a point where the system fails and fitness falls off a cliff edge. The fragility of the cannon bone in racehorses is a good example. Breeding for speed has made the bone longer, thinner, lighter, and so fragile that at the 2023 Kentucky Derby 16 horses failed.

 Strong selection for new mental abilities in the past 300,000 years gave humans extraordinary new cognitive performance but perhaps at the cost of failure for a few individuals. 

My article describing the idea generated a lot of  interest and I featured it in Chapter 14 of my recent book on evolutionary psychiatry and in an article in Psychology Today. 

But finally in 2024 Philipp Mitteroecker and Giuseppe Pierpaolo Merola have published a formal model and full discussion that I hope will move things along. 

Cliff-edged fitness functions can help to account for much more than broken horse legs and human mental vulnerabilities. Stay tuned.