[O]xytocin strengthened the biases of the Dutch volunteers. When they sniffed oxytocin (rather than the placebo), they were quicker to associate positive words with Dutch names than with either German or Arab ones.... [A]fter sniffing oxytocin, they were far less likely to sacrifice the Dutch loners than the German and Arab ones....Fascinating, but I'm not sure we need any group selectionism to understand this. It seems from the above that oxytocin nudges humans towards in-group loyalty on the trade-off between loyalty and universality. So in order to increase the fitness of individuals, oxytocin levels should tend to rise in situations when signaling loyalty is adaptive, and fall when signaling universality is adaptive.
This last experiment clearly shows a trend that applied to the whole study: oxytocin boosted favouritism for people who belong to the same group. Only very rarely did it increase negative feelings towards people outside it. For example, in the moral dilemmas, oxytocin made the volunteers less likely to sacrifice members of their own group, but not more likely to sacrifice outsiders.
This sort of favouritism makes a degree of evolutionary sense. It could bolster trust and cooperation within a community, such that groups whose members stuck together more would out-compete those that did not.... But such preferential treatment has an obvious dark side – it leads to all sorts of moral and cultural problems, including inequality, discrimination, prejudice, and conflicts between different groups.
Hmmm... do you think it is more adaptive to signal loyalty or universality during and immediately after sex? Call me prude, but I doubt you'd boost pair bonding by telling someone that you love them, but you love everyone else just as much. Then again, I s'pose that's an empirical question.