Evolutionary Explanations of Behavior
Some people are pretty hostile to evolutionary explanations of human behavior. So why should we believe them, or not? The most obvious reason to believe that some human behavioral tendencies are written in our genes comes from the fact that we that those of other animals are. There is also the fact that some behaviors are universal or nearly universal and seem to arise spontaneously in groups of people when they aren't provided by the wider culture. It's pretty obvious, though, that the cultural matrix we find ourselves in does have a big influence on human behavior, and that there is a wide range of behaviors among individuals and between cultures. It's clearly not viable to attribute all human behavior to instinct or all differences in behavior to genetic or other biological differences.
One good but surprising example of a behavior that has clear genetic roots is language. We don't all speak the same language, and the language(s) we learn are those we are immersed in as children, so how can I claim that language is an instinct? The best reason is the evidence of some remarkable natural experiments. The one I find most persuasive is the experience of deaf children who were housed together in a school, without access to either external deaf languages nor the language of speech, who showed up at school with only a few primitive and idiosyncratic (not mutually intelligible) gestures learned from their parents and spontaneously constructed a full featured and gramatically sophisticated sign language on their own. Other examples come from the creation of creoles. People of diverse linguistic backgrounds coming together will communicate with each other in primitive pidgins - small vocabularies with hardly any grammatic structure, but their children will make those into a full-featured and grammatical creole - a real language.
The lesson is very clear - if children don't have a language, or lack mutually intelligible languages, they create them from scratch or whastever pieces that are lying around, even if they can't hear. That instinct is clearly very flexible. The languages created will be original creation, differing in every surface detail of sound pattern or gesture and even many grammatical aspects, but they will be powerfully expressive and have a core of deep grammatical stucture. It should be expected that other human social instincts should be similarly flexible and adaptible to circumstance.
So when should a behavior be suspected of being genetic? Universality or widespread commonality is the first clue. Behaviors that we share with our close animal cousins are another hot candidate. When we look at individual as opposed to group behaviors, behaviors that run in families are good candidates t. These are not ironclad guarantees, of course, but then we must consider alternative explanations. Are there any? Do they stand up to scrutiny?