In some species, males help take care of their offspring. This is common in birds (~80%); uncommon in mammals (~6%), but it does occur – in wolves, for example.
In particular, humans exhibit paternal care, although the extent varies.
There are indications that this behavior is not purely cultural, not just learned, but has a biological substrate. Married men, and especially men with children, have lower testosterone levels, and it seems that this is causal: marriage possibly, and children probably, drive the reduction. At this point we don’t know the exact triggers, or the biochemical mechanism. This pattern of testosterone reduction associating with paternal care is seen in many other species, and it might be that studying some of those other species would help us figure this out. However, since gorillas don’t show much paternal care, while chimpanzees show none at all, it seems likely that paternal care arose independently in humans. There’s no guarantee that our mechanism is the same as that in wolves.
If we understood how this works, we might find that individuals and populations vary in their propensity to show paternal care ( for genetic reasons). I would guess that paternal care was ancestral in modern humans, but it’s easy enough to lose something like this when selective pressures no longer favor it. Wolves have paternal care, but dogs have lost it.
This could have something to do with better health in married men. High testosterone levels aren’t cost-free.
It’s possible that various modern environmental factors interfere with the triggers for dadliness. That would hardly be surprising, since we don’t really know how they work.
All this has a number of interesting social implications. Let’s see how many of them you guys can spot.