The reason monogomy is important on your account is because it reduces the level of competition/violence inside a group. Yet, it seems to be that the reason that (open) non-monogomy is on the rise is largely because the violence has been so effectively controlled.

If we ever start to slide back to a state where men fight duels because someone stole their girlfriend people will return to greater monogomy.

Also don’t most studies point to a direction of monogamous pairing with lots of cheating. Wouldn’t requiring paternity tests also change the game in “unnatural” ways.

If the issue is men ending up single, shouldn’t we promote polyamorous relationships or some form of polyandry?

But is it monogamy as such or stable families? In a sense, if every male know which females are his and which are his neighbour’s that would also make it so much easier to maintain peace and produce the claimed adaptations.

Its many things, but simply having each male know which females are his leads to disaster in a 50/50 male/female population. Take an extreme example- our society has decided that only men with a net worth of > 10 million dollars can reproduce. Women may freely choose any man to mate within this group and every other man works to support this situation. On the surface this seems like a plausible way to select for the traits that make successful individuals- but you get massive gambling addicts in the future. What happens is that any steady, hard working lower class person has a near zero chance of passing on his genes while his crazy, compulsive gambling neighbor could win the lottery/go on a hot run in vegas/win the trifecta and get to pass his genes along. Gambling becomes a far better strategy and it gets passed on into the gene pool.

That sounds like a bizarre and unrealistic example- but in a 50/50 society polygamy automatically leaves explicit losers in the gene leaving race (its even worse when males can reproduce indefinitely but women have a cut off point). If there are 50 males and 50 females and 1 male has 2 wives then 1 male has to have none. Then that last male has a huge incentive to steal a wife, or murder another man, and those kinds of activities create conflict. If 1 man has 10 wives then there are 9 buys out there with tons of sexual energy and all the genetic incentive to start murdering/stealing wives.

Ants and other eusocial species have an even higher reproductive skew. But they are true group selectionists with separate reproductive and working castes. Workers in a eusocial species have no option of diverting energy into the reproductive arena. Thus they work hard for the group because all other choices have essentially been permanently removed from the table.

Monogamy is a different (and more egalitarian) method of accomplishing the same goal. They both have very strong ex ante boundaries of reproductive access. For members of a working caste in a eusocial species the boundary is “you will never reproduce”. For monogamy the boundary is one partner. But either way, if these ex ante boundaries are strong then people will work at the group level rather than becoming status-seeking free riders.