Debating with a friend and the subject of twins separated at birth was brought up, and it occurred to me that as often as I hear about such studies, I never read anything about one directly.
I think that any conclusion one might draw from 200 people wouldn’t be statistically significant, and the mention of comparing the deviation in IQ between two twins, and one twin and a random person shows that the two twins are more similar, but is a random person really a good control in this instance. Wouldn’t a more informative control be a person of the same gender, born on the same day, in the same geographic area? The environment experienced during gestation has a definite influence on a child’s development, if it didn’t, we’d tell pregnant women to drink and smoke as much as they want.
Any better twins separated at birth studies? Any significant findings? Because it all sounds like unscientific crap to me.
l0k1 twin studies are an extremely hot and productive topic in behavioral ecology and related fields, because they let us track heritability of traits, and let us assess how much of the variation in a trait is attributable to genetic variation. (for most psychological, cognitive and behavioral traits, the answer seems to be ‘a lot’.) I don’t know why the most recent study you quote is from 1990, but Bouchard himself has done a lot more work with twins since then, and so have many others.
One of the best approaches is to compare identical and fraternal (dizygotic) twins, because then you can, in theory, remove maternal/prenatal effects. (In theory, dizygotic twins should share prenatal environment and around 50% of their genes, while identicals share closer to 100%).
A good study on this is Olson et al. “The heritability of attitudes.”
A substantial number of ‘identical twins reared apart’ studies have addressed the critical question of nature vs nature in things such as obesity and diabetes (both types I and II).
I think it’s fair to say that said studies have had a profound influence on the development of the current paradigm for the above disorders. In other words, without them, we would have a much poorer and less informed sense as to their aetiology.
Cites are easy to locate. Go to www.pubmed.com and search for (identical twins (diabetes or obesity)). For a more manageable number of results, restrict your search to ‘reviews’ (but you’ll still get over 100 results; and those are just the review articles).
Just read the abstract of the Olson study. It doesn’t mention anything about twins separated at birth, just 195 pairs of identical twins, and 141 pairs of fraternal same sex twins. I’m not interested in twins studies in general, but in twins separated at birth studies.
I don’t see any twins separated at birth studies, having to due with diabetes or anything else at pubmed. I see lots of twin studies, and I understand the reason for using fraternal twins as a control group in twin studies. My argument is that I often hear about twins separated at birth, but I don’t see any actual research on it other than the two studies I mentioned in my initial post. Perhaps I’m not using the correct naming convention when searching, but I’ve found studies anything but easy to locate.
For people with identical genomes raised in variably different environments, how do you decide which of them is the “control”? Seems to me at best you could cull a bunch of coincidences (i.e. both as children had dogs named Spot) or possibly make some generalized observation and hypothesis (the twin raised in the U.S. is significantly taller than the twin raised in China - suggesting the effects of better nutrition).
What’s your null hypothesis? “By studying twins raised apart, I hope to demonstrate that …” what?
“Separated at birth” twin studies are going to be rare because the population in minuscule. First, the number of twins is not really great, (currently around 33 per 1000 live births, giving us about a five million pairs in the U.S.). Adoptions currently run around 150,000 a year, (and the number is growing, so going back in time, each year would have had fewer). If we want fifty years of data we could begin 20 years ago, (to ensure that they are adults) and go back 50 years. Using the unrealistic figure of 150,000 adoptions a year, we come to a figure of seven and a half million persons. Using our 33 per 1,000 birth rate, we reduce eligible candidates to an inflated figure of fewer than 250,000. Then one has to find twins where some family trauma actually required that they, (or one of them), be removed from the birth home. Finally, you have to actually discover that a “separated at birth” twins event occurred and track down the twins affected–many, perhaps most of whom may be unaware of their situation.
On top of that, the (rare) practice of separating twins for adoption has been actively argued against for several decades, making the likelihood of studying such persons even less likely in the U.S. and other Western countries.
Such studies probably are rare, but few as they are, they are still going to be the best available. (I am not sure where you hear a lot about them, since they are, indeed, rare. More than intelligence, I would think that they give clues regarding personality, where separated twins grow up to take very similar jobs, marry people who look very much alike, display the same sexual orientation, follow similar politics, pursue similar levels of education, etc.)
I think back in the day, ‘twins separated at birth’ was a big deal because you could separate out effects of genetics and environment (since twins are nearly genetically identical). Nowadays there are more sophisticated ways to address the question, like comparisons of identical vs. fraternal twins.
I don’t have a null hypothesis. I’m not trying to prove anything about twins separated at birth. I contend that there are no twin separated at birth studies than the two I mentioned. One was so ethically questionable and had such a vanishingly small sample size (13 kinds) that the lead author decided not to publish and to seal the records until 2066 (or something). The other one mostly deals with IQ, which has questionable scientific value from my reading.
I’m asking if anyone can site any twin separated birth studies that have any valid conclusions about anything, other than two people who are very similar are more likely to score similar on an IQ test than two random people.
Perhaps it’s just me, but I often hear people reference twins separated at birth studies as if there have been lots of detailed scientific research and certain conclusions have been drawn. That sounds like crap to me. And I guess it is, since no one is contradicting me.
Uh, you don’t. You use someone else as the control. If twin A and twin B, who are monozygotic, and raised apart, both have a dog named Spot, the control would be twin A, and some random guy C. If Spot is a very popular dog name, then C might have that dog, and thus the fact that B also had that dog is not significant.
The unpublished study I’ve referenced was led by a guy named Peter B. Neubauer. He partnered with an adoption agency in New York, splitting up 5 pairs of twins, and one pair of triplets, in order to study them. He claimed, though never really cited, a scientific consensus of placing twins for adoption separately. He also made a lot of weird Freudian conclusions about certain people in the study. The small amount of his research that I read seems hugely anachronistic. You would swear it was either 19th century penis-envy, or some poorly written Hollywood version of psychiatric research.
Except that before DNA testing, a lot of twins were thought to be identical who are now understood to be fraternal but very similar in appearance, so identical twin studies of yore are suspect.
Sample size isn’t the only consideration. “Power” is the ability to find significance, but only if it is actually true in reality. You can do many artificial manipulations, or in this case you’d use a more powerful design that assumes a smaller source of error.
Statistical significance only is or isn’t true. You could find significance, but your study is crap because it says that eating broccoli is associated with 0.0001% higher IQ. If the effect is so naturally powerful already, it will be easier to recognize this with significance.
In other words, you can find real significance with 2 subjects, and that’s perfectly cromulent research. You’re right that studies involving individual differences like this probably need a few more subjects. But on the face of it you can’t tell if it’s sufficient or not without more information.
One issue is that 2 randoms = no genetic link siblings = 50% genetic links, but fraternal twins are more alike because they grew up at the same time. And identical twins are identically genetically (mostly), but would’ve had some environmental differences, and if similarities still exist you can chalk it up to genetics. Of course that isn’t completely true as identical twins are often treated as more alike than fraternals, particularly if opposite sex (may dress the same, have same friends, etc.).
And a matched pair of two people in the same environment is used - but it doesn’t make sense if you are explicitly trying to demonstrate a genetic link!
They’re their own controls. They probably treat them as the same person for analysis, which gives you more power (see above). They also do similar for paired studies, e.g. each half of a couple or age/SES/etc. matched individuals.
Sure it does, as my dog analogy illustrates. If you compare a set of twins separated at birth, and find lots of similarities (like in the Jim Lewis and Jim Springer case), more similarities than if you compare one twin to one random person, you might believe that you uncovered some genetic basis for having a wife named Linda.
But a random person is just that, random, and two people, born on the same day in the same city are not random. If we were to compare Jim Lewis to some guy 20 years younger, born in Nigeria, then you probably wouldn’t find that many similarities, however, if you compared Jim Lewis to some other guy born in Minnesota in 1940, it might not be that surprising if he also married a woman named Linda, since it was the second most popular girl’s name of the era.