Darwin was of the opinion that most behaviors we emit are from the "inside" and not affected much by the environment (in his Autobiography, and Descent of Man). The "Nature vs Nurture" was pretty much all Nature to him. I second that opinion from my observations of humans-that most of what humans do is because of hard wired stuff in their genes. The thing to add is that some behaviors will be collectively decided by the genes-not individually.
A word like Culture is eventually traceable to the genetic makeup of individuals. It is the collective behavior of the dominant or majority gene pool in a population. The dominant and majority population may not be the same (a small group might dominate the whole population). See my post on the theory of dominant genes before for more on this.
When people talk about French Culture, Mayan Culture, etc. there is a clear message of a genetic signature. 'Country' is the just another word for 'Tribe'-you can think of countries as very large tribes. Just as a small tribe of humans in Africa or Brazilian jungles has genes which are common between them (they often marry within their families), the same think is true for larger tribes called countries. Obviously migration of new people, intermarriage with other tribes, etc. slowly will change the genetic makeup of the tribe, but the fact at the bottom remains-there is a genetic commonality in a tribe or a country.
Naturally, we expect tribes or countries physically next to each other to have similar genetic makeups (cultures). This is true for all animals. So Germans are close to French people in their genetic makeup, etc. Obviously Spain is different from Morocco-but the point I want to make that *in general* geographical proximity will lead to similarity in genetic makeup of individuals.
Unscientific words like "Culture" and "Nurture" will I slowly be understood in hard scientific terms-what we called Culture will be proved to be genetically based behavior of certain prominent individuals of the group. There is little room for random/operant behavior. Much like the movement of heavenly bodies before Kepler and Netwon arrived was not well understand and perceived by many people as largely random, the same will be for a large amount of human and animal behavior. We will be able eventually to predict all behavior of humans, especially collective behavior of tribes and countries, by a thorough understanding of Genetics. Complex systems like living organisms are even more rule based then non living systems like planetary movements, geology and weather.
Social scientists will do well to have a very thorough understanding of Biology and Genetics. Whenever they start with the asumption that a human is some special animal, they are starting with en egocentric, erroneous assumption ( pointed out by Darwin in many of his works-he was very critical of people thinking animals as stupid, etc.). If humans are just looked at as another animal, things start to make more sense. And social science (psychology, sociology, political science) is standing on hollow ground without understanding biology and genetics. Granted there are empirical observations about behavior of individuals and societies which we can make, Economics by Adam Smith is one good example; but the real basis of these behaviors is Genetic. In that sense, even Economics and economic behavior of humans will be ultimately traceable to genetics. When Adam Smith says, the division of labor is effected by a human's ability to truck, barter and exchange, which seems to be innate in him (or speech and reason, which are obviously innate), he is really talking about the genetic cause of this behavior of trucking, bartering and exchange.
See also: Genetic affinity in friendships
Thoughts on mate selection in humans