Regarding the heritability of personality traits. Wow, someone is wrong on the Internet!
So, I saw a remark on YouTube that stated that “if we took two identical twins, and put them in two different households one very positive and one very negative, that the two would turn out differently despite having the same genes–and that therefore environment has the dominant role in the development of personality.” They also stated that this was “common sense”
There’s a problem with this. It’s not right. Not entirely. It is true for some aspects of personality, and not for others. People don’t like to hear this. The thing that bothers me about it is that this was a comment in response to one of my own that simply stated “environment matters less than you think”.
Here’s the quote: ” If you take twins and each of them are raised in seperate households. One is constantly reaffirmed and treated well, in a happy family. While the other is constantly belittled, and abused in a dysfunctional abusive family. You must understand that one twin will most likely have many more positive traits than the other.”
Now I thought that if you knew about actual twin studies, as this person said they were familiar with them, and knew anything about the heritability of personality and behavioral traits, that you would have moved past this point in your understanding. Calling out a single case that’s an hypothetical anecdotal one at that, doesn’t disprove what we know from the actual research that’s been done on the topic. It’s an interesting thought experiment. Hey, I do those sometimes too.
As I thought about it, of course I did see where the person was coming from.
I’m not saying that people are deterministically affected by their genes. I’m not saying that environment doesn’t have an effect. I’m not saying that it isn’t the dominant effect for this or that dimension of personality. It is, for some facets. Evidence suggests has slightly more influence for most of them. What I said, was that “environment matters less than you think.” Not sure why that sounds like “environment doesn’t matter” or whatever.
So. Let’s take a look at heritability in regards to personality and behavior to make sure you understand what we’re talking about. The following is from the University of Colorado:
“The concept of heritability plays a central role in the psychology of individual differences. Heritability has two definitions. The first is a statistical definition, and it defines heritability as the proportion of phenotypic variance in a population attributable to genetic variance. The second definition is more common-sensical. It defines heritability as the extent to which genetic individual differences contribute to
individual differences in observed behavior (or what’s called phenotypic individual differences). You should memorize both of these definitions.
Because heritability is a proportion, its numerical value will range from 0.0 (genes do not contribute at all to phenotypic individual differences) to 1.0 (genes are the only reason for individual differences). For human behavior, almost all estimates of heritability are in the moderate range of .30 to .60. [note that .6]
The quantity one minus heritability gives the environmental component of the trait. “Environmentability” [sic] has an analogous interpretation to heritability. It is the proportion of phenotypic variance attributable to environmental variance or the extent to which individual differences in the environment contribute to individual differences in behavior. If the heritability of most human behaviors is in the range of .30 to .60, then the environmentability of most human behaviors will be in the range of .40 to .70.
There are five important attributes about estimates of heritability and environmentability. They are:
Heritability and environmentability are abstract concepts. No matter what the numbers are, heritability estimates tell us nothing about the specific genes that contribute to a trait. Similarly, a numerical estimate of environmentability provides no information about the important environmental variables that influence a behavior.
Heritability and environmentability are population concepts. They tell us nothing about a specific individual.
A heritability of .40 informs us that, on average, about 40% of the individual differences that we observe in, say, shyness are in some way attributable to genetic individual difference. It does NOT mean that 40% of any specific person’s shyness is due to his/her genes and the other 60% is due to his/her environment.
Heritability depends on the range of typical environments in the population that is studied. If the environment of the population is fairly uniform, then heritability may be high, but if the range of environmental differences is very large, then heritability may be low. In different words, if everyone is treated the same environmentally, then any differences that we observe will largely be due to genes; heritability will be large in this case. However, if the environment treats people very differently, then heritability may be low. [this seems to be where the commenter was going in their thought experiment, which suggests an implicit assumption on their part, maybe, that environmental differences are very large in the human populations. Their example is a distortion of the concept–one that pushes environmental influence to the fore.]
Environmentability depends on the range of genotypes in the population studied. This is the converse of the point made above.
Heritability is no cause for therapeutic nihilism. Because heritability depends on the range of typical environments in the population studied, it tells us little about the extreme environmental interventions utilized in some therapies.”
This supports my counterpoint to the claim that environment (implicitly *always*) plays a greater role and that, as you see, fully supports my statements about genes vs environment. You can see that by looking at the extreme case, that is the *very extreme* case, where you have overridden the effects of their genes.
The average, the normal, the general case is what we’re concerned about and what we see when we observe the population as a whole, becuase when looking at it for a population as a whole, that’s what we get–the general case as an average of all the various types of environments. The extreme cases aren’t common, or they wouldn’t be extremes. People do establish themselves in notably dissimilar environments in some ways. But, the environments are, in terms of a broad sense, able to support human beings. There aren’t a ton of desert dwellers nor a ton of Eskimos. Note that we’re talking about, in this person’s example, serious abuse, deprivation, etc. is a very extreme case and just doesn’t apply to hertiability in a population study. It’s a nice thought experiment, but it doesn’t describe what we’re talking about. As we said above, that scenario applies to that particular individual. If somehow that scenario became the general case, where children were treated either one way, or the other, that would be rather different. Intellectual honesty tells us that it’s not the average case.
Now again, I do see where this person is coming from, it almost makes sense. If you aren’t familiar with the science, and are just thinking through it in your head. If you haven’t studied the field, you’d just think you were right. One way people tend to look at it is like it would be with cloned plants, but it’s a poor metaphor for something like personality. This one comes up a lot. The plants have identical DNA, but one is raised with poor light and soil. It’s scraggly and small. The other is raised in optimal conditions. It’s big and strong!
The thing is, the one raised in optimal conditions won’t go beyond the maximum attributes coded for by its DNA. Very poor nutrition might make one twin not as tall as the other if they were raised separately, but excellent nutrition isn’t going to make the twin that got that nutrition 10 feet tall. They’ll grow as tall as their genes code for them to be.
So, with the plants, maybe they both get the same amount of light and fertilizer, but one gets more of a trace element, or you pinch the nodes on one. Not severe damage, just a little. For those, one may be bushier than the other. The bushy one is only going to be as bushy as it’s genes allow. But it will be different. This is a better analogy I feel. It shows that of course environment makes a difference, and it can surely make significant differences–but the organism isn’t going to go “out of bounds” on how it can potentially develop.
Animal and especially human behavior is complicated. We need to leave this analogy and move on. So, have I proved this other person right, wrong? Anything? I don’t think so. I never said, remember, that there was no effect, or only a small effect. I said that environment mattered less than you think. When positing these examples, the danger in drawing conclusions is the same as in that person’s original “common sense” conclusion.
Note that the science on this heritability of personality traits is pretty easy to look up, and really ought to have given that person pause, when they saw numbers like the ranges published, specifically that .6. I’d think anyone even having the conversation would recognize that that’s more than half, and therefore is the “dominant” influence in the general case, for that aspect of personality.
Yet again, I never said that overall, for all aspects, that environment was the lesser influence, that it didn’t matter, that it was deterministic, or anything remotely like that. What I said, once again, and I hope it’s clear now, is that:
Environment matters less than you (probably) think.
And next, let’s look at a twin study.
Heritability of the big five personality dimensions and their facets: a twin study.
Jang KL1, Livesley WJ, Vernon PA.
The genetic and environmental etiology of the five-factor model of personality as measured by the revised NEO Personality Inventory (NEO-PI-R) was assessed using 123 pairs of identical twins and 127 pairs of fraternal twins. Broad genetic influence on the five dimensions of Neuroticism, Extraversion, Openness, Agreeableness, and Conscientiousness was estimated at Neuroticism 41%, Extraversion 53%, Openness 61%, Agreeableness 41%, and Conscientiousness at 44%, respectively. The facet scales also showed substantial heritability, although for several facets the genetic influence was largely nonadditive. The influence of the environment was consistent across all dimensions and facets. Shared environmental influences accounted for a negligible proportion of the variance in most scales, whereas nonshared environmental influences accounted for the majority of the environmental variance in all scales.
Small N, but this is all the way back in 1996, tons of research has been done since then that backs this up.
Now come on. Really. Think about that. Those numbers are higher than you likely thought, if you weren’t already aware of them, no? Note that two of the five are over fifty percent, which of course means that they are the dominant factor, where genes matter MORE than environment! (Once again, in terms of the population, not the individual) The others are all pretty close. The rest are in the 40’s.
Look: Openness is 61%. Wow. Unless you already knew that, I’m pretty sure that’s an effect that’s “more than you would think.”