Due to time and proximity from my work on past studies, I can't post for reference the studies done as of yet. Unfortunately, my impromptu search on the web for the theory isn't proving helpful at the moment... That said, as with many things in human history, there are many facets to historic changes. The sex equality is a compelling argument, and not what I'd subscribe to as the major facet.
From what I studied, with the rise of agriculture and sedentary populations, humanity also experienced many more strifes as consequence. Conflict over land, chronic disease, nutrient deficiencies - the last couple of which can be easily expounded upon here. In fact, once I get back home I hope to post a bit more on that, too. The whole subject is very interesting on what agriculture did to our bodies themselves, but I digress.
The main focus here that was touched upon in lecture was the emergence of war and competition for land and resources during the Neolithic era (around the time of agriculture showing up). As you can read at the wiki site in the previous paragraph, you can see that agriculture was shitty with regard to nutrition. Furthermore, agriculturists can work for longer hours with not only less results, but sometimes for naught.
The study, published in the journal Science, set out to investigate the apparent paradox that while people in hunter-gatherer societies show strong preferences for living with family members, in practice the groups they live in tend to comprise few closely related individuals.
Those tied to the now profession of farming allows others free time to fill alternative roles in their society as populations grow; whereas populations grow, families become larger and allow for offspring to contribute to farming.
The social organization of mobile hunter-gatherers has several derived features, including low within-camp relatedness and fluid meta-groups.
I would interpret that this implies agriculturists are not (which, of course, being my interpretation seems to further the theory taught) . Hence, a social stratification ensues - I believe it fits well with the theory that sex equality shifts at this point. All of this serves the base for war as a tool to usurp others' resources (food, land, workers) for another's gain and survival. Enter: the natural law, to some level, the amassing of power to some form of leadership to in a form of security contract in order to ensure the well-being of both the lands and farmers working the land ... not to mention the villagers/peoples themselves. Populations rise, social stratifications increase (due to wealth, less social mobility due to specialized roles), and soon fiefdom doesn't look to shabby.
It irks me to no end that I can't find the evidence that I'm searching for on the web right now. I'll reply to this comment once I get back home with the sources. I hope some of this is good food for thought, and I'd love to read of other theories or ideas!