Many people say that having children gives you a much better sense of the power of genes in shaping behavior. At least in the abstract sense that is not true in my case. I accept the "conventional wisdom" from behavior genetics that "shared environment" (colloquially, parental input) is relatively marginal in effecting much long term change within reason (i.e., if you don't beat your kid over the head with a baseball bat and such you don't have much influence). To review, on many bio-behavioral traits the different choices parents make seem to account for on the order of ~10 percent of the differences you see in the world out there amongst their (biological) offspring. Of the remainder of the variation about half of it is attributable to variation in genes, and the other half to unaccounted for non-shared environment. In The Nurture Assumption Judith Rich Harris proposes that that last effect can be reduced down to social environment or peer groups. Her line of argument is such that parents are important because of the genes they contribute, and, the environmental milieus which they select for their offspring. On one level I find this banal to review. If it is not the orthodoxy, this position seems relatively uncontroversial, and the results fall out of the data with minimal manipulation. But as a society such facts have simply not been internalized. In the great framing of "nature vs. nurture," appealing in its stylistic dichotomy, but not even wrong in its substance, the past few centuries have seen multiple swings between each stylized extreme. That has been a matter of ideology, not science. The popularity of public intellectuals such as Steven Pinker by the turn of the 20th century indicates to me that the high tide of post-World War II nurture-über alles has receded. But the media and popular culture are to some extent lagging indicators. They continue to trumpet correlations between parental choices and offspring outcomes as if there is a causal connection without pausing to consider the possibility both might be being influenced by a confound, genes. Confusions as at the link above are probably why I still blog and talk about this issue incessantly. It matters in our lives, and the current state of public and personal choice simply does not take into account the real structural conditions as they manifest in the world around us. Even among many people with a biological science background who in the abstract understand genes in all their conceptual and biophysical glory there is often a concrete mystification as to the power of genes to shape behavior across the generations. This is why I get so excited when public intellectuals with some influence and following, such as Jonah Lehrer, seem to take for granted basic, but often counter-intuitive inferences from behavior genetics. For example, if you generate a perfectly egalitarian society in terms of environmental inputs, the remaining variation will be driven by genes, rendering genetic variation paramount in explaining the patterns you see around us. I'm willing to bet money that Lehrer disagrees with me on broader political philosophies, as well as detailed policies (I'm one of the few science bloggers who is a self-identified conservative). But ultimately agreeing on facts is far more important for me than the policies which we may derive from those facts (that in itself is a normative position of course!). Which brings me back to how genetics relates to family. I have two younger brothers. One of them is rather close to my age. This is the brother with whom I grew up. We had very much the same shared environment. The other brother though is quite a bit younger. I was off at university before he was old enough for preschool. Though nominally we shared the same environment, my parents, the reality is that my parents moved, they aged many years, and their values shifted. As brothers we all do resemble each other physically, and to some extent cognitively (I am by far the least mathematical unfortunately). But, it is my youngest brother to whom I exhibit the closest physical match. And perhaps more importantly, in terms of personality, and political and religious beliefs, it is also he who resembles me more. But that's not the strangest aspect of our resemblance. When I first did bloggingheads.tv I had a confused thought that I was impersonating my youngest brother, because our mannerisms, way of speaking, etc., were just so surprisingly similar. Though I'd been told this before, I hadn't understood the peculiarity of it. The upshot of all of this is to explain that shared and non-shared environment do little to explain why two brothers who didn't grow up together both seem to be inexplicably attracted to paleolibertarianism and move their heads in the same manner when speaking. Especially when there' s a third brother who seems to be a shared and non-shared environment "control" with one of the brothers, and does not exhibit those behavioral traits. Finally, in terms of behavioral ticks I have a tendency to pick at my fingers in a very specific way. This a bad habit I've struggled with over my whole life. My mother has the same problem. And her mother had the same problem. The behavior isn't exotic or mysterious. Like many people we pick at our fingers when we are anxious. So I'll leave you with this: a few days ago my daughter apparently exhibited the exact same tick which I have, which my mother has, and which my late maternal grandmother had. I'm 99.99% sure that this 2 month old baby did not observe me picking at my fingers; when I'm with her (which is unfortunately infrequently because of my schedule) all my worries melt away. Obviously there's no "picking at your fingers" gene. But the phenomenon just goes to show how deep rooted some behaviors can be. * Before comments clarify, I am aware that this unaccounted for fraction may itself be genetic in the form of epistasis, or perhaps biological due to simple developmental stochasticity.