Was discussing the book "The selfish gene" with saurabh and we felt that it is in human nature to try to spread out as much as possible. And we tried to fathom how society became monogamous - contrary to basic human instincts.
So well earlier we were all hunters and life was risky. The more the number of people in the tribe the more powerful it would be and hence everyone was free for all. And the society was polygamous and polyandrous.
But then we became agriculture based. In early days - the size of land was not a constraint but the number of people working for it was. So the religion in those times expected woman to rear children. So women would start having kids since the age of 15. And they got dedicated to one man and settled with him - rearing his children. The polyandry bit was dropped and society became polygamous. However increasingly the agriculture income was not sufficient enough to support multiple wives and her kids so some religions like hinduism and christianity started having monogamous family. Though majority of religions still endorsed polygamy. But however, till our parents and our gen we have became monogamous.
Well now again both men and women are economically independent and need not really rear children. This is another extreme of from where we started. But the need to not having children or being loyal (for the sake of economic needs) is turning the society back to polygamous and polyandrous - a little unofficially.