It is impossible to give a completely precise explanation of how any person gets to be the way that they are. Even if we could measure the position of every atom in your body, this wouldn't tell us much useful about who you are, only really obvious things like how tall you are or how much you weigh. We humans are particularly interested in people's minds. What is your personality like? Do you have a good sense of humor? Can you play ping-pong? Our premise of reducibility and emergence says that all those things that are part of your mind must exist at the atomic level in some form, but even if we knew how those things are represented (which we don't), the complexity would be more than a human could understand.
Science is never about finding the entire precise truth (which is impossible anyway). Instead, it is about finding some level of analysis where we can discover an approximation of the truth that we can understand.
One thing that most people are interested in is the ways in which people are similar or different. It is easy to see this Social Comparison all around us, and Evolutionary Psychology offers explanations of why we are interested in these comparisons. Two broad questions we can ask about similarity and difference between people are:
In our people-watching, we sometimes wonder: Why is that person like that? Did someone deliberately teach them to act that way? Did they pick it up by imitating people they knew? Their parents or children in their neighborhood? Or were they just born that way? In science, why is the most important question because science is about finding out how things work. We want to know why. Why do we humans have in common the things that we do, and why do we differ in so many ways?
It's obvious that many of the things that we have in common with other humans are already that way when we are born. No one would say that a baby learned to have ten fingers–that trait is innate.
Even before scientific investigations of why people are the way they are, we already knew that parents are a very important influence. Human parents always have a human baby rather than a starling or a fern, and there is often a noticeable family resemblance between the child and both parents. Something about the parent's individual natures is inherited by the child. We have also long used selective breeding of farm crops and livestock to dramatically transform the innate nature of these living things, even without knowing anything about how this inherited essence is passed on.
We now know that this heritable essence is encoded in the structure of DNA molecules, and we know a great deal about how DNA is copied from parent to child and how this DNA is used to generate the particular structure of individual cells. Although we can describe many details of prenatal development, our understanding of the way in which DNA shapes the resulting person is very sketchy. Developmental biology often studies simple organisms such as worms and fruit flies because even these much simpler developmental processes are poorly understood. So we don't know how our DNA directs our development, but we have good reasons for saying that this is true (see Genetic Causes).
Now that we know the mechanism of heredity, what can we say about the causes of human similarity and human diversity? When we directly measure genetic diversity, we find that DNA is on average 99.9% similar between two modern humans. All human genetic variation falls in that remaining 0.1% of genetic diversity. Human DNA is also 99.7% similar between modern humans and Neanderthals and 96% similar between humans and chimpanzees, so small DNA differences can have large effects. Because all living humans are descended from a much smaller founding population only about 150,000 years ago (see out of Africa), all humans are relatively genetically similar, when compared to the diversity of other species that have been around for longer.
This genetic similarity is an obvious explanation for the similarities in appearance and behavior of people all around the world. Of course today we have global travel and communication which spread culture around the world, and even in pre-modern times people still did move around. Many similarities of European languages are due to the common influence of the Proto-Indo-European language, not genetics. But it is likely that the Cultural universals we see around the world arise as the best solutions to common problems faced in the human condition. In addition to our shared DNA, the human condition includes other universals, such as the physical world we live on. Human societies must find lifeways that “work” given these constraints, which tends to guide human behavior down certain paths.
When we look at the 0.1% of DNA that varies between people around the world, we find that 85% of this genetic variation is present within every population, while only 8% occurs between continents. Given our ignorance about which changes in DNA matter and which ones don't, we have to make do with the assumption that on average all DNA changes are equally relevant to the kinds of differences that we care about. Then we'd expect the genetic differences between people in any region to be about ten times larger than the average differences between populations in different regions. So it is much easier to uncover useful approximate truths about how “people are innately different” than it is to say approximately true things about how “Africans are innately different from south Asians”.
We know that humans have a good bit of genetic diversity, but does this diversity matter? We can't answer this question unless we pick out a specific thing that matters and have some way to measure that trait. For example, height certainly matters if you want to be a basketball player, and height is easy to measure. Creativity is also important in many things we do, but the only measures we have are very crude and indirect, such as the number of books an author publishes. In order to get to “step 1” in a scientific understanding of human diversity you have to come up with ways to turn human differences into numbers.
All human genetic variation amounts to only an 0.1% difference in our genetic code. In genetics we sometimes say that siblings “share half of their genes”, but what that really means is that they share half of their diversity. So siblings are expected to be at least 99.95% genetically similar.
Identical twins are clones. Their DNA is identical (except for a scattering of new mutations.) It is obvious that identical twins are quite similar in many ways, including height and facial features. When we say that identical twins are similar, we need to say: “compared to what?” We need to compare the differences between identical twins to the differences between people who aren't twins. For example, we could compare the similarity of identical twins (DNA 100% similar) to the similarity of two randomly chosen people (DNA 99.9% similar). By comparing these two measurements of similarity, we can estimate the heritability of that trait. Heritability is the percentage of individual trait diversity caused by inherited genetic diversity.
Of course, most children are raised by their parents, and parents give their children far more than just their DNA. The similarity between siblings is partly caused by their having been raised in the same family. Is there any way we can separate the (sub-)cultural and economic effects of the shared family environment from the genetic effects of shared DNA? Both effects could explain the fact that siblings are more similar than people taken at random.
One way to directly test the effect of shared family environment in comparison to genetics is to look at children where the connection between family and genetics has been broken. Adults often adopt unrelated children into their families. If differences in how families raise children were much more important in causing diversity than genetics, then we would expect that adopted children to be no more different from the other children in the family than the related children are from each other. In fact, for most measurable behavior traits, adopted children are much more similar to their biological relatives than to their adopted families.
Identical twins are also sometimes adopted into different families. This gives a direct test of the relative power of genetics and of family environment to shape behavior patterns. Identical twins being adopted apart is rare, which reduces the amount we can learn from these natural experiments, but heritability estimates based on measurements of twins raised apart are similar to the estimates from other sources.
A third way to estimate heritability (the most commonly used) is to compare the similarity between identical twins to the similarity of fraternal twins. This approach, the classical twin study, requires larger assumptions about how genetics and family environment interact, but allows the use of the much larger population of non-adopted identical twins, and also avoids weaknesses specific to adoption studies. Once again, this approach gives similar heritability estimates to the other techniques.
The specific way that heritability is estimated in a twin study gets complicated, but the idea is that if the diversity of family environments has a much bigger effect on trait diversity than the genetic diversity between families does, then there will be little difference between being identical twins and being fraternal twins. If we see greater similarity between identical twins than between fraternal twins, then this is the effect of genetics. One assumption is that the effect of the genetic difference between fraternal twins is analogous to the effect of the (different) genetic difference between families.
As children grow they also enter into a larger society, and learn from other people in addition to their parents. Many similarities between individuals are caused by shared Culture.