I couldn't find a good pithy blog post explaining it, so instead I'll direct you to a couple things I did find. I first read about it here (search for 'cave'): http://www.arthurdevany.com/2005/06/weepy_relatives.html

Which led me to Randolph Nesse: http://www-personal.umich.edu/~nesse/ who wrote this book: http://www.amazon.com/Why-We-Get-Sick-Darwinian/dp/067974674...