I call bullsh* on that one.
How can biceps circumference relate so strongly with evolution when nowadays bulging muscle is largely a result of gym training and nutrition (which, btw, costs money, biasing this study even further)?
I would say it's more about ego relating to muscle mass than the other way around. Egotistical rich men are more likely to be focused on their appearance and willing to turn into gym rats.
All I'm saying is that rich, "alpha" male are in the best position to frequent gyms heavily and bulge muscle. They have the drive, the time and the money. I think the study reached at this data, then (IMHO, wrongly) concludes there's a correlation (strong men choose competition over collaboration) instead of a causation (egotistical rich men have bigger biceps).
Also, biceps circumference only relates to fitness or strength up to a certain point, hypertrophy is left to bodybuilders. Just look at the average tribesman for examples of extreme fit people with average body measures.
A better experiment for this hypothesis would be to take these original subjects and have some of them adopt an exercise system and others to abandon one. Then test if ideologies actually change as a result of physical strength.
"These associations remained significant even once the researchers controlled for political party."
This seems to rule out the correlation coming from manual labor being associated with conservatism.
'men's biceps predict economically rational self-interest'