If you look at Robin Dunbar's work, he would claim that the limit for social groups to effectively manage their own 'alturism' is ~150 people. About the size if a village, corporate department, amish community or similar entity.
He proposed that humans have a cognitive limit, based on the size of our brains - though he is an antropologist, not a neurologist, so we can assume his info on human settlements is more accurate than his assumptions about brain structures.
The idea is that these kinds of social groupings function on a form of peer pressure.
In corprations everyone needs a certain work ethic, and with peer pressure you can ensure that everyone pulls their own weight with only natural social pressures - and most corps don't have depts larger than 150 (or 130-170, cause +/-20 is a fair error margin) because that is when this natural social pressures break down.
Talk to anyone who grew up in a village where everyone knows everyone, and contrast with people who grew up in cities. (Also you will see that suicide rates increase dramatically when communities break down - whether that is rural communities being de-populated by outward migration, or urban centers not including people, social exclusion is one pretty major factor in suicide - what we might call an illness of civilisation, or we could talk about societal health)
So no, greed is not necessarily the most signifigant drive for humans. Unfortunately scale is a pretty signifigant issue.
There is no point in talking about democracy, capitalism or communism without recognising that the scale of the system will effect the overall societal health. (And FYI, i personally think that the extreme partisanship in the US is a sign of an unhealthy democratic system - Iceland, by virtue of difference in scale, seems much healthier).