Identical twins: The differences explored

We are told (in a Nova program, Ghost in your Genes, October 16, 2007) ,

Scientists have long puzzled over the different fates of identical twins: both have the same genes, yet only one may develop a serious disease like cancer or autism. What's going on? Does something else besides genes determine who we are? In this program, NOVA reveals the clues that have led scientists to a new picture of genetic control and expression. One such clue is the surprisingly modest number of genes that turned up when technology made it possible to map the human genome. The Human Genome Project was originally expected to find at least 100,000 genes defining the human species. Instead the effort yielded only about 20,000—about the same number as in fish or mice—too few, some believe, to account for human complexity. Learn more about the connection between epigenetics, aging, and cancer on the program's companion website.

"What's going on? Does something else besides genes determine who we are?"

Um, yes. Here are three obvious observations right away:

- All we need to know about any life form is not necessarily in its DNA, as the program makes clear. Frustratingly, the true causes and cures of cancer and autism are controversial and clouded.

But our DNA is not a book of magic in which all the answers are written, and it is too bad if anyone thought it was.

- Identical twins may have almost-identical DNA, but usually one is the dominant twin and the other the sub-dominant one. Also, they tend to separate as adults and have different experiences. Over a lifetime, these differences can add up.

- Also, humans are intelligent and make independent choices. Different choices lead to different outcomes. The fact that anyone should doubt this is a symptom of the damage materialism (= you are either a robot or a monkey) has done to science.

See also Identical twins does not mean identical minds

Intelligence: How much is heredity and how much is environment

How much brain do you need?

Hat tip: Stephanie West Allen at Brains on Purpose

Identical twins: Identical genes does not mean identical minds

Jonah Leher, at Frontal Cortex, briefly profiles identical twins who are poets with vastly different styles:

... the Dickman twins, who were raised together and have been close their entire lives, seem to offer a parallel experiment. One way of looking at their work - Michael's Dickinsonian severity, and Matthew's Whitmanesque expansiveness - is as an illustration of the distinctiveness of imagination, even in two people who are as alike as two people can be.

Yes indeed.

I wish some authors (no reference to Lehrer) would get over the idea that we should expect identical twins to have similar personalities. For one thing, one usually becomes the dominant twin and the other the recessive one. That changes personality dramatically, and I imagine that writing style might differ as a result.

Another possible influence on personality is whether twins are socially rewarded for acting similar (even when they don't feel similar). When I was a child in the 1950s, some parents liked to dress identical twins identically. They thought that was really cute. Wiser heads condemned the practice, of course, and - from what I can see - it has lost cultural favour, at least in my part of Toronto.

Also, in some schools, identical twins might be assigned to different classrooms rather than side by side - on the grounds that language and social skills develop faster. In that case, the twins could be experiencing very different early environments as well. (That was done when I was a child, perhaps as a reaction to the parents who thought that dressing identicals exactly alike was really cute.)

See also: Intelligence: How much is heredity and how much is environment?

Once again: How much brain do you need?

Hat tip: Stephanie West Allen at Brains on Purpose

Intelligence: How much is heredity and how much environment?

Double helix DNA pioneer James Watson had to resign his job recently over remarks about race and intelligence. Recently, I learned about a finding on intelligence that might prompt people to rethink their approach to it - the Flynn effect. As Richard Restak explains it in American Scholar,

Flynn’s most intriguing and controversial claim concerns the preponderant influence of the environment over genetic inheritance in determining intelligence. The direct effect of genes on IQ accounts for only 36 percent of IQ variance, Flynn tells us, with 64 percent resulting from the indirect effect of genes plus environmental differences uncorrelated with genes. Yet this cheeky claim would seem to be contradicted by the fact that identical twins separated at birth and raised apart end up with very similar IQs, presumably because of their identical genes. Not so, says Flynn, who buttresses his argument by drawing on an analogy from basketball.

If on the basis of their genetic inheritance, separated-twin pairs are tall, quick, and athletically inclined, both members are likely to be interested in basketball, practice assiduously, play better, and eventually attract the attention of basketball coaches capable of transforming them into world-class competitors. Other twin pairs, in contrast, endowed with shared genes that predispose them to be shorter and stodgier than average will display little aptitude or enthusiasm for playing basketball and will end up as spectators rather than as players.

Well, that’s basketball. What about IQ?

“Genetic advantages that may have been quite modest at birth have a huge effect on eventual basketball skills by getting matched with better environments,” Flynn writes. He suggests a similar environmental influence on genetic inheritance in regard to IQ: Twins with even a slight genetic IQ advantage are more likely to be drawn toward learning, perform better during their primary and secondary education, and thereby gain acceptance into top-tier universities. In the process, their IQ levels are likely to increase even further.

According to Flynn, the environment will always be the principal determinant of whether or not a particular genetic predisposition gets to be fully expressed. “There is a strong tendency for a genetic advantage or disadvantage to get more and more matched to a corresponding environment,” he writes.

In other words, the focus on genetic inheritance is misplaced if it encourages people to look for smart genes, at the expense of looking for environments in which they may be expressed.

While we’re here, one thing has always bugged me about the stories of “identical twins raised apart” who turned out to be similar: While they do provide strong evidence for genetic inheritance of some characteristics, it may not be as strong as some think. When social workers must find homes for identical twins, they don’t send one to live with a drug dealer down on the waterfront and the other to live with a mad prophet up on a lonely mountain. They place children with conventional people who can’t have kids. So the people who get to adopt babies are not a random sample of humanity. They are people whom social workers expect to be responsible parents. They are also people willing to undergo the hassle of home studies and the endless waits for a baby (and also unwilling or unable to just raise $50,000 cash and hit the black or grey market).

I am not knocking identical twin studies. I just think that we shouldn’t discount the “who do social workers think would make a good parent?” effect in determining outcomes. And remember, it was usually the same social workers making the decision for both of the twins ...