Innate or Culturally Acquired?
One of the more contentious issues in anthropology is that of whether certain traits are rooted in biology or culture. This is sometimes called the “nature versus nurture” debate. My experience is that many seemingly irreconcilable differences turn out to be rooted in different definitions of terms. One complication of the nature/nurture debate is that due to the relatively extreme (compared to other animals) plasticity of human behavior, most behaviors clearly have roots in both nature and nurture.
Let me start with my definition: To be considered innate, a trait needs to be rooted in biology and heredity. To be culturally acquired it needs to be learned by imitation or teaching. Humans (almost always) walk upright, unlike our chimpanzee cousins. We have muscle and bone adaptations to facilitate upright posture and gait. Of course we almost always have the opportunity in childhood to watch upright walking, so there might be a learned component as well.
Like many other human behaviors, walking and talking are not present at birth. There is clearly a learned component. On the other hand, there is every evidence that we are pre-programmed to learn to walk and talk. A whole range of activities in preparation for these activities spontaneously start occurring at more or less predictable stages of an infant's development.
Language is an example that illustrates the full complexity. An isolated human infant will not develop language by itself, though she will go through most of the linguistic preparatory stages. If she misses out on communication at the crucial developmental stages, she won't be able to catch up later – language will be stunted or absent. Almost all of us learn the language we are exposed to as infants. Fascinatingly, however, unlike an isolated individual, groups of children without a common language do develop a common language, and a full-featured grammatically rich language at that. That strongly suggests, or actually insists, that the mental substructure underlying language and core grammatical principles is built into our brains.
When can you be sure that an aspect of behavior is not innate? The obvious answer is when it isn't universal. Speaking Spanish is not innate, because only people exposed to it, or taught it, can. Speaking is innate, since almost everybody can (absent some crippling physical or mental abnormalities). The existence of such abnormalities is another strong piece of evidence for innateness. Is the converse true? Does universality imply innateness? Not quite, but it's extremely strong presumptive evidence. It's at least conceivable that a universal human behavior has been transmitted culturally down the generations. On the other hand, spontaneous appearance of the behavior in the absence of a cultural model (like the creation of languages de novo) are about as strong a form of evidence as one can imagine.
Our most interesting collective human behaviors (morality, religion, politics) are the most complex of all. Here again there is plenty of evidence of innateness but so much plasticity that drawing the lines is difficult.