Sex at Dawn: The Prehistoric Origins of Modern Sexuality by husband and wife psychologists Christopher Ryan and Cacilda Jethá (2010, Harper). Another readable and very interesting book (albeit with a few flaws).
Their basic argument is that our ancestors were sexually promiscuous, much like our close evolutionary relatives the bonobos (Pan paniscus). In hunter-gatherer societies, the connection between sex and the birth of babies was not clear and, they contend, children were seen as communal property shared by the entire tribe. Men and women both had multi-male and multi-female partners and sexual relations strengthened the ties between members of the community. Ryan and Jethá provide a number of anecdotes from still extent hunter-gatherer societies to bolster their arguments.
They argue that the concept of sexual monogamy only developed in the past few thousand years with the advent of agriculture. Why? Because with agriculture came settled life, ownership of land, and, with the domestication of animals, the awareness that babies have both a mother and a father (farmers learned to selectively breed animals). Paternity became important because land was left to offspring. Patriarchal monotheism arose in the same place the earliest cities and farmers developed (the Fertile Crescent) teaching that monogamy was God's will and that women were essentially the property of men.
This books thesis is in direct conflict to many who have argued that humans are "naturally" monogamous. The facts on the ground, however, don't seem to support our cultural concept of the idealized nuclear family - half of divorces occur because of infidelity, pornography has a strong appeal to men who spent, in 2006, an astounding $97 billion dollars on the industry, and an estimated 1 in 25 dads are raising children they did not father.
Ryan and Jethá try to make their case by comparing us to the other hominid apes (our closest ancestors) - talking about such scintillating topics as ape sex, penis and testicle size comparisons, sperm competition, copulation frequency, and others. Interesting stuff. Convincing? I'm not sure.
There are problems. One gets the impression reading this book that it's a polemic and contrary evidence is not presented or discussed in any meaningful way. Anecdotes are often presented in support of claims and the book is repetitive in places. It's not a scientific work.
It is, however, an interesting book and, I believe, mostly correct. As a species, we're not naturally monogamous. That doesn't neccesarily mean we need to give into our baser instincts to be horndogs, there are certainly benefits to a traditional nuclear family - especially when children are involved. But, perhaps it means that we need to be more forgiving of those who can't quite live up the the idealization.