New World primates have higher glucocorticoid levels and lower glucocorticoid receptor sensitivity than Old World primates.

“New World monkeys monkeys in South America marmosets tamarins or whatever when you compare them to traditional old-world monkeys the Asian the african once turns out they have like an order of magnitude higher glucocorticoid levels oh my god that's crazy what's that about all sorts of theorize about how for some reason it's more stressful to be a new world primate than an old world which didn't make a whole lot of sense and then there was this issue of these animals should have been falling out of trees from their very Cushing's disease and and finally somebody found an explanation no somewhere back when there was a mutation in the gene for the glucocorticoid receptor in new world primates and as roughly 1/10 the sensitivity so the system just equilibrated out in a different set point”