Well, capitalism in various sociohistorical moments is not a "homogenized ensemble of social relations", so it should make sense that psychological characteristics vary somewhat in different capitalist societies at different times. I do have a hard time imagining how a capitalist society could function by producing people who focus on the common good rather than self-interest. That would be a bit of a poison pill for any capitalist society, wouldn't it? (CEO: "Hey, we could make a killing doing X, but I've discovered that X would have a negative effect on our society as a whole, so I'm not doing X. Sorry, no dividends this year." Shareholder: "Great! I'm going to sell my stock in more profitable companies and buy more stock in your company!")
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> There are vast differences in the degree of atomism versus social cohesion, individualism versus communalism, and these don't necessarily fall along the lines of capitalist as opposed to pre-capitalist societies. These differences are reflected, for instance, in the vast disparities in rates of violence in crime amongst industrialized nations - differences which do not easily correlate to poverty or wealth disparities, especially once we include industrialized East Asia.
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Sure, lots of factors other than capitalist social relations shape social life and people's psychological characteristics. The point I'm making here is that these observed differences are the result of social conditions and social structure, not simply individuals' psychological characteristics. (Explaining why one society is more violent than another by reference to the aggressive/violent personalities of specific individuals is a category mistake through and through.)
Miles