This is an interesting topic that may border on the uncomfortable for many. As much as Disney and Hollywood want to convince you, monogamy was not our natural state of relationship with one another.
There are a number of theories about how monogamy came into existence. I’ll skirt over the moral and ethical details, as they are highly subjective, and just lay out the “facts”.
To being with, I need to clarify that as we are looking back to find the root of monogamy, it no longer becomes “human” monogamy, but rather monogamy as a wider species. To this end, here are a few statistics to keep in mind: monogamy in primates is found in 15% of species compared with about 5% for mammals as a whole (Schaik and Dunbar, 1990). It also invariably involves close spatial association between the members of the pair (sorry guys, long distance is a no-no; more on that another time).
Monogamy in these species did not evolve because males are unable to defend access to more than one female. Hence, it must be related to behavioural services provided by the male which substantially increases the female’s reproductive output.
The major proposal by Schaik and Dunbar is that the services mentioned in the quote above involve protection against predators, defence of an exclusive feeding area, and protection of the female and child against infanticide by other males. This paper is a bit old but keep in mind the main points: protection and food.
Where resources are transferred across generations, social monogamy can be advantageous if partitioning of resources among the offspring of multiple wives causes a depletion of their fitness value and/or if females grant husbands higher fidelity in exchange for exclusive investment of resources in their offspring. This may explain why monogamous marriage prevailed among the historical societies of Eurasia: here, intensive agriculture led to scarcity of land, with depletion in the value of estates through partitioning among multiple heirs.
The major point here is that social monogamy is the outcome of strategic behaviour in regards to the allocation of resources to the next generation (Fortunato and Archetti, 2010). Again, we can see a common theme. Not necessarily food, but resources/wealth (which are essentially the same as food when considering the difference between humans and other animals).
Finally, we have one of the most recent studies done by Sergey Gavrilets at the University of Tennessee (2012). Gavrilets identifies a key trait in all polygamous alpha males: they don’t have to invest in their young because they’ll have plenty of offspring regardless. By comparison, a supportive male (not an alpha, but helps provide food and protection), can also be successful reproductively speaking, but only if they can be certain of their “target” children – otherwise they will be wasting resources on offspring that aren’t their own. Using the complex mathematics of his field (biomathematics), he reconciled a model for the transition from alpha polygamy to our current social paradigm of monogamy (a paradigm that is again turning a bit due to the rambunctious youths).
The crux of the matter? Low-ranking males offered food to females in return for mating opportunities as they had no prospects in physical domination. Obviously, these males were more likely to select faithful females. And think about it, women love material things right? Even gold-diggers will stay faithful (or appear to be) to a lesser male if the promise of wealth is large enough. This evolution into monogamy also signified a change in the concept of what “success” and “power” are. Today, they are more frequently associated with monetary assets than physical prowess.
It has been said, obviously, that Gavrilets’s paper is a bit oversimplified, but hey – spherical chickens in a vacuum (science joke; click the link for an explanation).
So what does this mean? Women are shallow? Well, yeah, but it’s in their nature. That’s how they evolved. No, seriously though, you can take away whatever you want from this information. Whether you hold patriarchal views or the more radical “modern” views, these are just scientific approaches to a social phenomenon. Biologically and evolutionarily speaking, I understand polygamy and the need to sleep around. I try not to judge people for it either, and consider myself quite open minded. Personally, though, in terms of a long-term partner I’m willing to really invest in, I’d prefer a faithful girl. Maybe that strips me of an alpha male status, but that’s how we appear to have evolved. Well, as it stands I probably have more physical prowess than any monetary assets so …