Stephen Beckerman of Penn State University and others have noted it in a number of traditional cultures. He suggests that children in such cultures fare better. He suggests a child of the Bari of Venezuela, is 16% more likely than a single-fathered child to survive to the age of 15, probably due to improved nutrition. Among the Aché people of Eastern Paraguay, having multiple fathers appears to protect children from violence, the main cause of infant and child mortality. Others however have asserted that children do less well than when specific fathers are tasked with the upkeep of specific children, particularly ones that they are persuaded that they have fathered themselves.
To suggest a resolution to the argument, given that such cultures are rare, and in-depth studies are lacking, we must necessarily speculate about possible environmental origins.
Firstly we predict that there will not be a marked difference in status between males. In many such groups, individuals may be closely related, siblings and cousins, giving strong familial bonds. Secondly in a difficult environment with a high mortality rate, no child will be left without a father. It may be in those cultures where social stratification is starting to appear that partible paternity becomes less effective.
The practice may have been more widespread in the long-distant past than we realise. In "The Gallic Wars", Book one, Chapter 14, Julius Caesar writes about the Celts who inhabited Kent in England: Ten and even twelve have wives common to them, and particularly brothers among brothers, and parents among their children; but if there be any issue by these wives, they are reputed to be the children of those by whom respectively each was first espoused when a virgin.
This may be, and probably has been, read as the women of the tribe being the communal property of the males. It could just as well be read as the women, in what may have been a matrifocal culture, ensuring the best support for their children. It is interesting to note that Bressan suggests that human babies do not resemble either parent, and women in our culture go to great lengths to convince fathers of their paternity.