The monogamous nuclear family only emerged with Capitalism. Before Capitalism, traditional, tribal societies were classless and they practised a form of 'primitive communism' in which there was no private property. With the emergence of Capitalism, society and the family changed. Capitalism is based on a system of private ownership. Eventually, the Bourgeois started to look for ways to pass on their wealth to the next generation, rather than having it shared out amongst the masses, and this is where the monogamous nuclear family comes from. It is the best way of guaranteeing that you are passing on your property to your son, because in a monogamous relationship you have a clear idea of who your own children are. Ultimately, this arrangement reproduces inequality as the children of the rich grow up into wealth, while the children of the poor remain poor