Heritability and why Parents (but not Parenting) Matter

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Synopsis

To be blunt, most of what you think you know regarding why parents matter is very likely wrong.

This article is written by Brian Boutwell and Razib Khan

“…Who in the world am I? Ah, that’s the great puzzle.”

— Alice from Alice’s Adventures in Wonderland by Lewis Carroll

Like Alice, we’ve all pondered the question: “who am I?”  Moreover, we often couple it with the reasonable companion query: “how did I get to be this way.”  Not all of us are rich and famous, we can’t all bend guitar strings like Hendrix, and most of us will never have supermodel looks or the physical prowess of a professional athlete.  There is fascinating unity in all of us, though, concerning how we answer the question of “why am I this way” as opposed to some other possible version of myself.  Whether we credit them for our successes, or point at them as a hurdle that we had to clear, most of us implicate our parents when constructing a narrative about why we are the way that we are.  It’s not an unreasonable intuition.  But how we intuit about the world can mislead us; sometimes that “light at the end of your tunnel” is, in reality, “just a freight train coming your way.”*1 To be blunt, most of what you think you know regarding why parents matter is very likely wrong.

You are the product of precisely two biological parents (even the so-called, “three-parent babies” are somewhat of a misnomer).  Whether you know them or not, whether you like them or not, the fact that you have parents (and were, in fact, not cloned from a prior version of yourself) carries with it great consequences.  Yet, family socialization effects on personality are not large, not prominent, and not pronounced (and for many traits, they are absent).¹ What parents do to their children (in particular, their style of parenting and efforts at socialization) does not leave permanent marks and does not differentiate individuals within the population for outcomes like intelligence, antisocial behavior, and a host of other outcomes.1,2,3 So why then, are parents consequential (beyond the obvious role that they play in providing safety, shelter, etc.)?  How do we rectify the apparent contradiction?

Let’s first discuss a term that has appeared frequently in our discussions: heritability. Heritability represents the proportion of variance in a particular trait that is explained by genetic variance within the population. As we’ve described before, heritability captures the role that genetic differences play in making some people (relative to others) taller or shorter, heavier or lighter, more intelligent or less intelligent, so on and so forth.1,2  Virtually every measurable trait (which varies within the population), we now know, is heritable. This reality has become remarkably unimpressive to many behavioral geneticists, though it continues to blindside some scientists who have managed not to pay attention for the last several decades.1,2,3  The shift among journal reviewers has been remarkable to witness, really, moving from: “there’s no way that trait X is heritable” to “of course trait X is heritable, who cares?” or even the more dubious (and disingenuous) “heritability is pointless, let’s talk about epigenetics [or gene-environment interaction]!”2,3 Changing sentiments aside, heritability continues to be an extremely relevant point for behavioral scientists to wrangle with.  Understanding why it is important, moreover, will take us a long way to understanding why parents (but not their parenting styles) are of such monumental importance in explaining variation in outcomes.

To complicate things just a bit, you should know that there are two types of heritability: narrow and broad sense.If you’ve ever heard the expression (perhaps from a plant or animal breeder) that a trait “breeds true” then you’re already informally acquainted with narrow-sense heritability.*2  Heritability in the narrow-sense captures variation that results from additive genetic influence.Heritability in the broad-sense includes other genetic effects which are non-additive (i.e., also in a technical sense, they can be non-linear).Imagine taking the effect of every gene that influences a trait—most of which would be very small—and adding them up; thus the term, “additive.”  Additive traits tend to breed true—meaning that variable characteristics manifest in the offspring in a similar (i.e., correlated) fashion to parents who possess such characteristics (at the population level and individual level).

Non-additive effects don’t breed true with the same degree of fidelity—in fact, these effects bust up the clean transmission of traits from parents to children (to use a sports analogy, it’s a bit like a cornerback in football deflecting the quarterback’s perfect spiral aimed at the receiver).  Two brown-eyed parents can give rise to blue-eyed children, for instance.  Prior to Brother Mendel (just in case you weren’t aware, he was a monk) ever fiddling with his first pea pod (peas; the bane of all childhood existences), breeders were keenly interested in whether they could breed for certain traits.  Could you make the pigs fatter, or could you make the crops more disease resistant or drought tolerant?  If the answer was yes, then wealth and fame was surely in your future.  Knowing something about the narrow-sense heritability of a trait was important in that day and time, and it remains that way today.

We’re treading on delicate ground here because any time you start to talk about breeding, genetics, and (gasp) humans, it conjures up demonic specters.2,3 As if summoned from some regrettable Ouija board session, the ghastly ghoul of state-sponsored breeding programs comes barging into the room.  We’ll assume that we’re all in agreement regarding the moral repugnance of coercive eugenics, thus stipulating that such programs should never (ever, ever, ad infinitum) return.  The thing is, humans already have their own breeding initiative, and they have for some time now.  If you don’t believe us, consider for yourself how you picked a mate.  Did it happen to be at random? No?  What possibly could have gone wrong with that approach?  Think about it another way; say that you signed up for a high dollar dating service.  After several dud dates, and a subsequent phone call with customer service, would you be pleased to find out that your dates were being picked by the complex matching algorithm known as flinging darts against a wall with photos tapped on it?  If your answer is yes, then please stop reading and let’s get you signed up for: www.Boutwell-KhanDatingServices.com; our motto is: We throw darts as randomly as anyone, and we offer coupons!

Individuals discriminate on a range of qualities when searching for a mate.5,6  Even arranged marriages can’t get around this reality.  The difference with an arranged marriage, of course, is that it is often a more “collective process”, with parents exhibiting increased control and discernment (perhaps more than the newly betrothed would like).  Regardless of who does the choosing (parents or children), it isn’t done haphazardly.  The qualities of emphasis might vary in some situations (for instance, whether the goal is short term liaison versus long term partnership, it might dictate what one is willing to tolerate in a partner)5 but the process is still far from random.  The qualities that humans prefer in their mates (what men look for in women and vice versa), moreover, are remarkably consistent across cultures.  There has been some excellent work on this topic carried out by evolutionary psychologists.5,6  Why does any of this matter?  Remember the issue of “breeding true”; well it applies to humans just like it applies to pea plants.  First, though, we need to talk more about sexual (and artificial) selection and foxes.

Famed biologist Richard Dawkins noted in his enjoyable (and important) book “The Greatest Show on Earth,7 that sexual (and artificial) selection offers powerful evidence in favor of Darwin’s arguments about natural selection.  It certainly does, and conveniently for our purposes, it also provides a very useful bit of insight.  Consider the landmark and very famous work (highlighted by Dawkins) carried out on the selective breeding of foxes.8 The goal of the work, in large part, was to better understand the processes of animal domestication. Foxes displaying high levels of tameness (when around a human) were bred with each other.  As it turns out, if you continually cross one tame fox with another, what you eventually get is an animal that starts to act (and even look) more like a dog than a fox.7,8  “Humans are not foxes!” objects the incredulous critic. True, but we don’t get to play by different rules (apologies to our creationist friends [and sadly, to many sociologists*3]).  The same evolutionary processes that apply to the fox and the hound (and every other organism on the planet), apply to us.7,8

Now we are getting more to the meat of the issue.  Only a few pieces of information are needed to know something about what is likely to happen to a quantitative trait over time that is selectively bred for in a population.9 All that you need is the narrow-sense heritability of the trait (h2), the mean for trait (or the average value of the trait in the population), and the values possessed by the parents on a certain trait, and you are in business.9 Positive assortment for an extreme value in a trait that is highly heritable  (e.g., tall people pairing up with other tall people) is a mechanism for altering the distribution of values for that trait in the population.  You obtain a greater number of very tall people (thus, shifting the mean for height upwards)!  This is largely why parents matter, and this is also why knowing something about the heritability of a trait remains important (see Boutwell’s first essay on the topic of parenting for a more methodological treatment of why h2 is relevant).

Consider an example that dovetails with the previous point.  Criminologists (and psychologists) have been aware for some time that criminal involvement runs in families*4 and is also heritable (a good portion of that heritability seems to be narrow-sense, though not all; see quillette.com/#comments).  The psychologist Robert Krueger and colleagues10 some years back, provided evidence that humans mate assortatively for antisocial and criminogenic behaviors (put differently, highly antisocial individuals tend to pair off with each other in a non-random fashion).  Does this completely explain the concentration of crime in certain families?  No.  Can it be safely ignored and assumed to be irrelevant?  No.  Just as narrow-sense heritability is a puzzle piece that can tell you something about where the distribution of a trait could be headed in a population, it also helps inform the question of why certain traits cluster in families.9

An observant reader will argue that heritability is not a fixed quantity; it is subject to change over time.  The finding that the narrow-sense heritability of most personality traits is about .50 (or slightly lower), for instance, does not mean that it has always been that way, or that it will always be that way.While true, this point doesn’t serve as the ammunition for refuting the relevance of narrow-sense heritability that some might think.  At any point in history, the environment might explain more (or less) of the variance in some trait.  Nonetheless, if that fact somehow negated the import of h2 then plant scientists and animal breeders should search for other ways to spend their time—breeding programs would be impossible (and they are not).  Now, if you listen even more closely, you can hear the murmur of critics ready to hurl the objection of “epigenetics” in our direction.  We knew it would come up; it inevitably does these days.  But you must never forget that epigenetics is not a “get out of jail free” card that grants us immunity or exemption from evolutionary processes.  It certainly does not make heritability obsolete.

If epigenetic effects are highly variable across generations, given that the statistical methods used by behavioral geneticists subdivide “environmental” effects from heritable effects, then epigenetics will be included into that parameter (i.e., one of the environmental parameters). Alternatively, if epigenetic effects are highly transmissible across generations, then they are to a large extent heritable (an important point in its own right that so many sideline enthusiasts gloss over).  This comes, of course, with the necessary caveat that strong environmental shocks can induce epigenetic changes.  Importantly, extant empirical research does not suggest (yet) that these sorts of heritable epigenetic dynamics are pervasive enough to be problematic for heritability estimates.  They likely exist, but they are the exceptions, not the rule.11

 

Over the course of two essays, I (Boutwell) have tried to walk you through a series of topics that I see as incredibly important in modern social science. The first dealt with problems in social science research (genetic confounding).  The largest takeaway point was simply that you cannot trust most research because most of it does not control for genetic influences on the outcomes they study (and we know that these effects are pervasive).  The second essay dealt with the complexity of ferreting out a parenting effect among the bewildering complexity of gene-environment correlation and interactions.  I also introduced you to the idea that epigenetics is unlikely to be the savior that many thought it was guaranteed to be.  For the third installment, Razib and I have tried to illustrate a fundamental evolutionary point: heritability matters.  Once again using parents as an exemplar.

Parents pass genes to their children.  In a Darwinian sense, this is of incredible importance.  Darwin, the brilliant scientist, prescient about so much, was nonetheless unaware that genes were the “packets of inheritance” responsible for making parents and progeny similar to one another.  It took the “religious” devotion of a monk (Mendel) to drive that point home.  Regarding those genes, Richard Dawkins poetically pointed out that (p.20):

They are in you and in me; they created us, body and mind; and their preservation is the ultimate rationale for our existence. They have come a long way, those replicators. Now they go by the name of genes, and we are their survival machines.12

Granted, this is probably not the defense of parents that you wanted, but then again, we never promised to deliver (necessarily) what you wanted.  We did intend to provide a “big-picture” rationale for why parents matter and why heritability does too.  In that regard, there is no better way to make the point then evolutionarily.  Parents matter because they pass genes from their generation to the next; that’s a pretty big deal in our estimation.

If what you were looking for is an impassioned crusade for why a certain approach to parenting is essential to civilized society, you won’t find that here.  There are times when it is best to let go of the feelings that you have regarding how the world works. In many cases, your self-assurance regarding your knowledge on some topic affords you little protection from science (anyone joining up for the Flat Earth Society these days?).  Famed biologist Robert Trivers strained our intuitions about the nature of parenting years ago by pointing out that the interests of mom and dad are not always the same as their filial creation (their child*5).  Behavior genetic studies were the flood that further obliterated the dam (for some context as to how parenting effects can be reconsidered, see my previous essay How to Find a Parenting Effect).  Parenting styles in the normal range of variation simply do not shed much insight into why people differ.

So that you don’t depart this parenting trilogy feeling too upset, though, consider a final point.  Your parents are important to you.  Do you really need to tack on the qualifier that they molded you like a ball of clay?  In order for you to appreciate your parents, must it be the case that they injected your personality directly into your head?  They didn’t, they injected it into your DNA (lovely image, right).  Your environment outside the home, as well as pure luck and happenstance shaped it from there.  You can still love and appreciate your parents, though.  Not one thing must change.  So as not to break with tradition from our first two parenting discussions, we leave you with the insights of someone else.  This time, Paul Simon has the last word.

“Still, a man hears what he wants to hear and disregards the rest.”*6

 

This article was originally published in Quilette 

 

Razib Khan is a writer and doctoral candidate in genomics at the University of California. Follow him on Twitter: @razibkhan

Brian Boutwell is an Associate Professor of Criminology and Criminal Justice at Saint Louis University. Follow him on Twitter: @fsnole1

 

*Footnotes

  1. Find us two other academics that can work Lewis Carroll and Metallica [No Leaf Clover] into the same discussion.
  2. See also; Pinker, S. (2002). The Blank Slate: The Modern Denial of Human Nature. New York: Viking.
  3. Horowitz, M., Yaworsky, W., & Kickham, K. (2014). Whither the blank slate? A report on the reception of evolutionary biological ideas among sociological theorists. Sociological Spectrum, 34(6), 489-509.
  4. For a striking example, see: Frisell, T., Lichtenstein, P., & Långström, N. (2011). Violent crime runs in families: a total population study of 12.5 million individuals. Psychological medicine, 41(01), 97-105.
  5. Trivers, R. L. (1974). Parent-offspring conflict. American zoologist, 14(1), 249- 264.
  6. Simon and Garfunkel; The Boxer

References

  1. Polderman, T. J., Benyamin, B., de Leeuw, C. A., Sullivan, P. F., van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature genetics47, 702–70.
  2.  Barnes, J. C., Wright, J. P., Boutwell, B. B., Schwartz, J. A., Connolly, E. J., Nedelec, L., & Beaver, K. M. (2014). Demonstrating the validity of twin research in criminology. Criminology52(4), 588-626.
  3. Wright, J. P., Barnes, J. C., Boutwell, B. B., Schwartz, J. A., Connolly, E. J., Nedelec, J. L., & Beaver, K. M. (2015). Mathematical proof is not minutiae and irreducible complexity is not a theory: a final response to Burt and Simons and a call to criminologists. Criminology53(1), 113-120.
  4. Plomin, Robert, John C. DeFries, Valerie S. Knopik, and Jenae M. Neiderhiser. 2013. Behavioral Genetics, 6th ed. New York: Worth.
  5. Buss, D. M. (1994). The evolution of desire: Strategies for human mating. New York, NY: Perseus.
  6. Buss, D. M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and brain sciences12(01), 1-14.
  7. Dawkins, R. (2009). The greatest show on earth: The evidence for evolution. Simon and Schuster.
  8.  Belyaev, D. K. (1979). Destabilizing selection as a factor in domestication. Journal of Heredity70(5), 301-308.
  9.  Snustad, D., & Simmons, M. (2009). Principles of genetics. 5th ed. New York: John Wiley & Sons
  10. Krueger, R. F., Moffitt, T. E., Caspi, A., Bleske, A., & Silva, P. A. (1998). Assortative mating for antisocial behavior: Developmental and methodological implications. Behavior Genetics28(3), 173-186.
  11. Moffitt, Terrie E., and Amber Beckley. Abandon twin research? Embrace epigenetic research? Premature advice for criminologists. Criminology 53.1 (2015): 121- 126.
  12. Dawkins, R. (1976). The selfish gene. Oxford: Oxford University Press.

Tags: brian boutwell, epigenetic effects, genetics, heritability, parenting effects, razib khan

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