How do you tell nature from nurture? (yeah, another 'men and women' thread)

[visceral]

How do you tell the difference between evolved traits and 'programmed' ones?

 

I'm thinking specifically of differences between the sexes here. For example I've read various things in 'science' books like The Female Brain (Louann Brizendine) like how women evolved to cry more easily, or that they find it much harder to keep their emotions to themselves.

 

How does anyone know these things are due to evolution? As a society we give females the right to cry but not males. We give females the right to be vulnerable, but not males. Girls and boys are treated differently while still wearing their amniotic membrane. Girls are mostly treated as just potential nurturers/encouraged to develop and interest in looking pretty. Boys are encouraged to develop visual-spatial skills ( as in using toy cars, building things, and the like.).

 

http://www.springerlink.com/content/p88566041u435g96

 

http://www.nytimes.com/1993/04/15/garden/parent-child.html?pagewanted=1

 

So how exactly do you determine what is environmental and what is written in their DNA?

[Cap'n Refsmmat]

Twin studies are a good start. First you find a bunch of identical twins (who have identical DNA) and see how much difference there is between them in various traits (like visual-spatial skills). Next, find fraternal twins (who only share about half of their DNA) and see how much difference there is between them. If there is more difference, it is likely that the trait is genetically determined.

 

I'd have to do some research to see what studies have been done.

[Paralith]

Firstly, most behavioral traits are probably a combination of both genes and environment, so it's doubtful you'll ever find one that's completely determined by just one or the other. And until the exact ontogenetic mechanisms are found, it really comes down to correlations, like the twin studies Cap'n Refsmmat described. If you look at 100 pairs of identical twins, with all the exact same DNA, and 90 of them have co-twins with differing phenotypes, then it's unlikely that the trait in question is genetically determined, or if it does have a genetic component it doesn't play a particularly large role. Another example might be that if the majority of humans in a wide variety of cultures and environments express the same phenotype, then the trait in question is more likely to be a result of our common ancestry than through chance. A lot of work on sex differences has been substantiated this way.

 

The other problem with complex behaviors is that it is pretty unlikely that there's only a single gene affecting it - there are probably many genes at work, some of which also play a part in other related traits, etc. Some papers have pretty convincing evidence that they have isolated at least one gene or gene region that is very likely affecting a given trait - but then the question is, to what extent? How? If there are unknown numbers of genes that affect the outcome in the trait as well as potential unknown environmental factors, this one isolate might only predict 2% of the variance in the trait, or something like that. Does that enable you to say much about whether or not the trait is mostly genetic or mostly environmental? Not really.

[KagakuOtaku]

Just a thought, here, but perhaps men try not to cry as easily because of the whole dominance thing. In most, if not all, species, the males tend to try to be more dominant over other males, yes? Why not in humans? Maybe we see the women as crying more, when it's actually the men who aren't crying very much.

[PhDwannabe]

Pretty good responses so far to a good (and important) question. Thought I'd throw in a small two cents.

 

Twin studies are a good start.

 

You used just the right words there, cap'n: "a good start." Many in psychology absolutely freak out about twin studies. Yes, they're the gold standard in determining what's inherited and what isn't (we call this "heritability"). But that doesn't mean they're perfect. Let me break down the three big types of twin studies in more exhaustive terms than cap'n did, though he already said much of this:

Identical Twin Studies: compare identical twins on your trait of interest. Say, schizophrenia, for instance. Find that the rate of concordance is something like 80%--when one has it, 80% of the time, the other one has it too. So we're done, right? 80%? Not yet. Not only do they share genes, they shared an environment. So, then we do...

Fraternal Twin Studies: theoretically, they shared the same environment, though they are only as genetically similar as brother and sister. This should allow us to distill out the relative influence of the two. Suppose that you study them and find out that when one has schizophrenia, the other one has it 30% of the time. 80 - 30 = 50% genetic.

 

Hold the phone, though. There's a big thing we're not ruling out, and that's fetal environment. Genetic influence is not the sum total of biological influence. Though the fraternal twins didn't share genes, they shared the same digs for 9 months, during which they were exposed to the same array of hormones, nutrition, and possibly diseases or teratogenic factors. So that 50% is really just non post-natal environmental influence--some of which is likely to be genetic, some of which isn't. There are other little issues with twin studies like these as well. Sometimes, if we're lucky, we get a large enough sample to study the granddaddy of heritability samples:

 

Identical Twins Raised Apart Studies: The postnatal environments are really different, but genes and prenatal environments are the same. This should tell us very, very precisely about the influence of a postnatal environment. This is the one people freak out about, and that gets a lot of fun media coverage. The body of studies often known collectively as the Minnesota Twin Family Study included a section like this: The Minnesota Study of Twins Reared Apart. Often, you hear stories about ridiculous similarities between long-lost twins--one well-known one you probably heard in gradeschool allegedly comes from the MSTRA: "One pair of twins had both divorced women named Linda and then married women named Betty. They later discovered that before they met each other as adults, they had taken several Florida vacations on the very same stretch of beach and had driven there in the same model of Chevrolet. They had both named their sons James Alan (one was "Allen") and both chain smoked Salems. Both chewed their nails and had woodworking shops in their basements." I'll save the speech about how human beings tend to remember and repeat amazing information like this, and forget about the million other times where nothing weird happens.

 

So, let me rain on the parade a little. One of the problems with them is that, to really use them as a perfect measure of heritability, one of the conditions you'd have to fulfill is random assignment. If we got to be evil research overlords and design this study ourselves, we'd separate the kids at birth and then randomly select couples somewhere in the world, and parachute in that kid with a note that says "we'll be checking up on you from time to time, thanks." But that's not what happens in these situations. Often, they're handled by families or localities, such that the two environments are, in essence, correlated. The kids get exposed to somewhat similar environments by virtue of the fact that they've stayed within a similar family, regional locality, and/or nation or culture. If none of the kids get thrown around between very different cultures (and they almost never do), we can't separate out the influence of culture. If indeed the famous set of twins above really existed as often reported (I've never really cared to check up on it), maybe part of the reason they both smoked Salems was hereditary in some convoluted way. But it was also because one of them wasn't adopted out to Yemen, where he may have chewed khat all day instead. It's because of ridiculous complications like these (and the fact that we can't experimentally separate children at birth... damn government) that it's very difficult to say what proportion of a trait is "driven by" genetic or other biological causes, and what part is driven by environmental ones.

 

But let me take yet another step back, and rain on the rain that's already raining on that parade. This entire discussion has been predicated (and is almost always predicated, no matter where you hear it talked about) on a largely unexamined assumption: that genetic/biological and environmental causes are proportions of a whole which add up to 100%. This is not true. To illustrate, let me give you a mathematical function:

 

f(x) = x² - 4x

x = 6

f(x) = 12

 

So, I gave you the function--the set of instructions, as it were--I have you the number to "plug into" those instructions, and then we got an answer. So tell me now: what proportion of the answer came from, or was "driven by" the function, and how much of it came from the value we plugged in? The question is insane. The two things don't produce anything without one another. They work together. Well, genes/biology and environment similarly work together. Each actualizes and works through the other. Genes create predispositions which alters the patterns of environments one is exposed to. Environments create conditions which influence the regulation of gene expression. These things are entwined in a way that can't be broken into two pieces that add up to 100%.

 

The heritability coefficients that we're so fond of ("intelligence is heritable at .5," we often say, meaning 50% of it comes from one's genetics/biology) simply do not tell us how much of a trait comes from that. They tell us how much came from genetics/biology in the sample studied, at the time it was studied, under the conditions of the study. It's a normative measure, not an absolute measure of the "power" of the gene. If we were to change that environment around, the heritability level would change! Certain environments allow, promote, or tamp down the expression of genes in different ways. Suppose we tested normal sets of twins and found that verbal skills were heritable at something like a .4--a decent enough result. Now, test another set of twins where one half of the set was raised chained up in a dark basement with no human interaction. Guess what? Your heritiability coefficient--what people take as a measure of exclusively the gene's power--is now at zero.

 

That's a dramatic example, but this works in real-life too. People use heritability coefficients politically to agitate against programs like Head Start. They say: "what's the use throwing all that money at these kids? Most of these skills are highly heritable, blah blah blah." Guess what? Change the kids' environment, and the heritabilty itself changes. Neato, eh?

 

This concludes my wandering, probably largely non-question-answering rant.

 

 

Thanks,

DJ

[jimmydasaint]

An anecdotal reply, which is by no means scientific or evidence of any sort. I had the good fortune of being to teach identical twin boys who 'evolved' to form different friendship groups in the same classes. One of the boys became a visual bias learner whilst the other had a kinaesthetic learning bias, by virtue of adopting the traits of their different groups of friends. I found it quite interesting at the time.

 

learning styles

[Marat]

A good way to sort out the differences between which female traits are caused by social conditioning and which are genetically caused is to look at women's behavior in different societies which have little or no connection with each other. The fact that women cry more readily in isolated Amazon Native tribes and also in New York City strongly suggests that this is genetic rather than cultural, unless by some astonishing coincidence Amazon Native and New York City culture both just happened to decide to condition female children to behave in the same way.

 

The evolutionary reason for this may be that since women are physically weaker than men (average hemoglobin of 120 in women vs. 140 in men, ranging all the way up to 170 in some men; much lower muscle mass and higher fat percentage in women than in men), it makes more sense for women to warn the tribe of danger by screaming and crying easily so that the men can hear them at a distance and come running to apply their muscle to address the emergency. This is the same evolutionary reason why children scream and cry more easily than adults.