...come immediately to mind.

In the environment in which humans evolved, it was not uncommon for women to die during childbirth. In fact, one can argue that the odds may have been as high as one in four or five for any given pregnancy. Yes, male mortality was also high (though not always so), but it would not exactly seem to be expected that males mating monogamously for life would have resulted from such a situation. Men had to get used to losing their mates/wives, and it goes without saying that Plan B involves another woman. So at least the potential had to be there.

The same could also be said for the common practice of tribes raiding their neighbors for women, because a guy could also lose his mate/wife this way, though its not clear which direction the causality arrow points with this. Is it the shortage of women which causes the theft and thus makes variety evolutionarily adaptive, or is it a predilection for variety which causes the theft in the first place?

Moreover, women in the distant past (and in many primitive cultures today) nurse their child for a much longer time than is currently the practice in the Modern West, often as long as 3-4 years. When you add in the relatively long gestation period in human females, well you may be talking around 5 years in what was then a rather short life as an adult of maybe only 15 years where it wouldn't have been functional for guys to sit around twiddling their thumbs rather than "getting busy", again with another woman. Women who are pregnant or nursing are not fertile, and it would not have been functional for men to find them sexy or attractive. All this may have been why polygamous systems evolved in so many places around the globe, as males who could took additional wives while an earlier one was not available for more reproduction related activities. The Pill making women fake being pregnant (in order to make any fertilized egg not implant) could be an additional important factor along these lines in today's world.

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