Double Trouble
February 22nd, 2008A few months ago I wrote about being an identical twin. Back then I’d done some research on twins, but I really didn’t find out that much. That was before I read “Entwined Lives”, a fantastic book that covers just about everything twin-related under the sun. It was even written by a twin, Nancy Segal. (She’s fraternal.)
Nancy (with a Ph.D. in twin research) writes about all kinds of twins: fraternal, identical, conjoined, and even triplets and other “multiples”. Considering that she’s a fraternal twin herself, I think it’s funny that she talks about the “lesser” status of fraternal twins compared to identical ones. Fraternal twins get a lot less attention, presumably because they usually don’t look alike and thus don’t generate a lot of interest and questions. As I mentioned before, being a fraternal twin is basically like having a brother or sister that’s the same age and has the same birthday. That adds a few wrinkles to the typical sibling story, but not that many. Plus, we’re rarer. (In fact, in the U.S. identical twins are about 5-6 times rarer than fraternal twins these days.)
“Entwined Lives” also covers a great deal of twin research that’s out there, which is where this blog comes in. The main question I want to focus on in this regard is an age-old one: Nature or nurture? Of course, my answer (and Nancy’s) is that, as usual, both matter. But it’s not a percentage breakdown. I agree with her that some aspects of ourselves are more determined by genetics, and others by experience. Plus nature and nurture form a feedback loop, I think, that makes the effects of each extremely difficult to tease out.
Despite this caveat, what things are more influenced by nature or nurture, then? In other words, where do genes play the biggest part? (I should mention that genes may not be the whole story, as recently biologists have discovered that RNA plays a big role, too. But for now, let’s ignore that since you probably get what I mean, and “genetically-identical twins” just doesn’t sound as cool.)
It turns out that Nancy outlines everything neatly in the final chapter of “Entwined Lives”, called “Double Entendre: Twinship’s Many Meanings”. It’s a wonderful and succinct summary of many twin-related findings, some of which she admits are buried in scholarly journals. I wonder if a non-twin would have been as thorough as her. Either way, I’m in her debt.
Anyway, the short answer is that “Twin studies support genetic influence upon nearly all measured behaviors (although love styles and mate choice are notable exceptions).” So Ed, you don’t have to worry about me stealing your girlfriends anytime soon. (Ed. note: That’s a relief.)
That sort of statement tends to rile up behaviorists like B.F. Skinner, though, who think nurture is everything. But Nancy also says that “non-shared” experiences account for a fair amount of behavioral differences as well. Curiously, shared family environments don’t seem to play much of a role. It’s what you learn when your family isn’t around that really seems to matter.
An important thing to remember in all this is that “individual differences characterize virtually every measured trait”. Which is to say that even identical twins differ at least a tiny bit on most things. Twins sometimes display “non-concordant behavior”, too, where one will get a disease that is known to have a genetic basis (like heart disease) and the other won’t. Depending on how strongly genes play into a particular behavior, this may be easier or harder to control. She also makes the point that some environmental behaviors are hard to change, even when you’re aware of them. To put it another way, our personal development is difficult to categorize with a simple dichotomy like nature vs. nurture and twin studies make that point clear.
But enough about that. Let’s talk specifics. How big a role do genes play? Intelligence, as measured by IQ, is strongly influenced by genes. (Though that doesn’t mean your genes determine how well you do in school. Many tests that are designed to test natural intellectual talent can actually be bested by hard work and studying.) Identical twins generally test within 10 IQ points of each other. I can’t tell you exactly what me and my brother got on our last IQ test, but rest assured it was pretty close. (Also, keep in mind everybody seems to do better on IQ tests over time.)
Oddly enough, twins generally do more poorly on intelligence tests than singletons, especially on verbal skills. My personal take on this is that twins rely on and understand each other so much that they don’t feel a need to communicate as often, and that they think more poorly when apart in testing situations (especially identical twins). Nancy mentions that twins are more likely to be abused (sadly) and get less attention from their families (as you’d expect), which can also play a part in intelligence tests. Compounding these effects are the fact that many twins are born prematurely (including me and Ed, as I mentioned in my last post). Premature births are associated with a host of problems, including lower IQ.
In this context, my twin brother Ed’s stints as a copy editor and typesetter are deeply ironic. (Ed. note: And helpful for this blog!) For my own part, I participated in a writer’s conference in second grade (my story was a crayon pictorial about a raccoon that played tennis) and I won a Christmas writing contest on TV in third grade. I got tickets to see a live-action Rumpelstiltskin, which I don’t think I went to. Plus I wrote at least 250,000 words in college. Oh, and my brother and I were both lucky enough to test well on IQ tests too. Take that, twin studies!
Next, personality is somewhat determined by genetics. Ed and I share a lot, but we often express our interest in slightly different ways. Apparently this is common with identical twins. Like both twins will be doctors, but choose different specializations. My brother is a programmer and I’m a business analyst, as it turns out. We both work in information technology, but in our own ways. In another life, maybe things would be different.
We definitely don’t have the same personality, though, which explains why we get into nasty fights sometimes. Thankfully, they usually don’t seem to last. Identical twins almost always seem to at least get along, according to research. It’s extremely rare that they won’t talk or do something crazy like try to kill each other. (Ed. note: Phew!) Even then, you’ll notice that the other twin asked for leniency from the court for her attempted murderer.
Twins seem to share this weird relationship where normal societal rules don’t seem to apply (more so with identical twins). I read about twin athletes in “Entwined Lives” that said they absolutely had to win, unless their twin beat them, which was OK. Can you imagine Peyton Manning (NFL quarterback) saying he had to win every week, but it was OK if Eli Manning (his brother) beat him? I doubt it. That said, I wonder how identical twin NFL athletes Tiki and Ronde Barber felt when one of their teams beat the other? (Tiki has since retired.) I couldn’t find anything saying one way or the other, but I’m guessing they didn’t mind so much, or it probably would’ve shown up in the news. (Another important question: Would the Giants have won the Super Bowl this year if Tiki was still on the Giants? Tiki would’ve had to beat his brother Ronde on the road in their first playoff game.)
Finally, genetics plays a smaller (but significant) role in things like choice of mate and job selection. I was surprised to read that the spouses of identical twins are frequently unattracted to their “cotwin”. Furthermore, most identical twins are often ambivalent about their twin’s mate, according to research. Personally, I refuse to comment on this whole situation, in the interest of diplomacy (and keeping my copy editor). I don’t think there’s a good answer I can give. So I won’t. What I can say is that I wouldn’t mind having my twin’s job as a programmer, though clearly that’s not how I ended up.
Obviously, then, nature and nurture play a big role, though depending on the behavior, one can be a bigger influence than the other. Combine that with feedback effects of both genes and environments (as I read, genes can effect environment and vice versa), and it’s clear why there’s no consensus response. It’s a complicated question, with a highly nuanced answer. I know that’s hard to get across in a short blog post like this, but at least this answer is accurate (according to research, anyway).
One thing is for certain, though - genes play a huge part in basic physical factors, like weight and height. Ed and I are within a half-inch of each other in height (apparently almost all identical twins are within 3 inches of each other). And our weights differ by maybe 5 pounds at most (most identical twins are within 10-15 pounds in weight).
Ironically enough, I was talking to a cashier at the grocery store today, who happens to be dating one of a pair of identical twins. She told me she doesn’t have to worry about them switching places on her. Why? Because one twin is apparently much heavier than the other. (She got the skinny one.) So even with the most basic of dimensions, people with the same genes can differ every once in awhile. Life is funny like that.
On a side note, should we even be called “identical twins”? Apparently it’s possible for us to have small genetic differences. I wonder if the more cautious phrase “99 percent identical twins” will ever catch on…
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February 22nd, 2008 at 1:14 am
I found your site on technorati and read a few of your other posts. Keep up the good work. I just added your RSS feed to my Google News Reader. Looking forward to reading more from you.
Tim Ramsey
I appreciate it, Tim. Glad to hear you’re enjoying the site!
- Dave
February 22nd, 2008 at 9:21 am
he short answer is that “Twin studies support genetic influence upon nearly all measured behaviors (although love styles and mate choice are notable exceptions).” So Ed, you don’t have to worry about me stealing your girlfriends anytime soon. (Ed. note: That’s a relief.)
So you don’t share your brother’s passion for Janet Reno, then?
Ha ha ha gross man, don’t remind me.
- Dave