Twin study

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Twin studies are studies conducted on human twins, usually involving identical (monozygotic, or MZ) twins in some way. These studies typically aim to infer how much of the variation in a human trait is due to genetic factors and how much is due to the environment. Most commonly, these studies use the classical twin method in which similarity (correlation) is calculated between identical twins for a given trait, and compared to the similarity between fraternal (dizygotic, or DZ) twins for the same trait. These two correlation coefficients are often then entered into Falconer's formula to generate a heritability estimate. (Falconer's formula states that the narrow-sense heritability, which is the percent of variation due to additive genetic factors, is equal to double the difference between the identical and fraternal twin correlations.) Falconer's formula is now regarded as having flawed assumptions. But there are other types of twin studies as well, notably the design involving identical twins reared apart (the idea is that they should have the same genes, but be raised in different environments). Perhaps the most famous of these is the "landmark" Minnesota Study of Twins Reared Apart that began in 1979. Of course, there are problems with these types of studies as well, including that "reared apart" is not necessarily the same as "reared in significantly different environments".

Limitations of twin studies

One fundamental limitation of twin studies is that, being observational studies, they cannot account for differences which do not exist (or do not exist in significant numbers of people) in the population, or in the sample of the population in the study. This includes both differences in effects and differences in causes (both environmental and genetic). Thus, even if a heritability estimate from a twin study indicates that a property is extremely heritable, it would be incorrect to conclude that no possible environmental intervention could affect it very much. We know that deliberate interventions like administering human growth hormone to short children can affect their height, but in a very poor population without much access to modern medicine, this environmental intervention might not be present and thus would not "show up" in a heritability estimate from a twin study performed on that population. Moreover, just because a characteristic is highly heritable now, it might not be in 100 years time, if a new medical intervention or technology is invented that can affect that characteristic, and that medical intervention or technology becomes widely used.

Also, on the other side of the nature versus nurture debate, it would clearly incorrect to take a heritability estimate from a twin study done on an isolated tribe as evidence supporting the (incorrect) belief that the ability to digest milk is highly non-heritable in all human beings, because it could just be that the gene for lactose tolerance is absent, or exists only in very low levels, in that population. This contrived example illustrates the danger in simplistic extrapolation of heritability findings from one human population to another very different population.

Notorious studies

Nazis

See the main articles on this topic: Nazi human experimentation and Nazism

The Nazis conducted experiments on twins under the direction of Josef Mengele. These studies were cruel, torturous, and often resulted in the twins' deaths. The experiments were incidentally without scientific merit.

The Neubauer experiment

Film poster for Three Identical Strangers

From 1960-1980, psychologist Peter B. Neubauer (1913–2008) led an investigation on twins separated at birth. The experiment involved collaboration with the Louise Wise Services adoption agency, which would find separate families for identical twin babies (and one set of triplets) given up for adoption. The adoptive parents would not be told that their adopted children had twin siblings, or that in many cases the birth mothers had had periods of severe mental illness. The adoptive parents were told that their children had been selected for a psychological study on childhood growth, but not the real motives of the experiment: that it was on twins, that it was an experiment on nature (genetic inheritance) vs. nurture, or the relationship between genetics and mental illness. Neubauer never published his results, or made public the motives or methods of the experiment. The twin study was terminated in 1980 when the triplets chanced upon one another in New York and briefly became celebrities: Neuberger must have known the jig was up.

The experiment had a limited form of informed consent insofar as the birth mother consented to giving up her children and the adoptive parents consented to take the children, something that would probably have been viewed as acceptable at the start of the experiment, must have seemed increasingly less so by the time the experiment was terminated.

The design problems with the experiment as they are now known were that Neubauer was a Freudian, a school of psychology that is largely pseudoscience. Experimental methods included giving the children dubious IQ tests, and Rorschach tests, which by 1959 were already known to be pseudoscientific.

One of the adoptive fathers said that the three triplet families requested a meeting with the adoption agency to try to get some answers. Immediately after meeting, the father returned to retrieve his umbrella only to discover the adoption agency officials celebrating that the meeting had gone so well. Further evidence of consciousness of guilt by Neubauer was his covenant on his records of the study donated to Yale University that they not be unsealed until 2065. After the release of the film Three Identical Strangers in 2007, the people controlling the secrecy of the records became sufficiently embarrassed to release a tranche of 10,000 redacted pages.

There is sort of a sick irony in the Neubauer study because he was a Jew who fled from Austria after it was annexed by Nazi Germany, Louise Wise Services primarily served Jewish families, and many if not all of the adoptive families were Jewish. An aunt in one of the adoptive families said that her family was persecuted in the Holocaust and briefly compared it to the Nazi twin experiment when expressing her grief about the Neubauer revelations.

Much of the presumed motivations for Neubauer's sub-scientific study have subsequently been surpassed by actual science that is not tainted by minimally-informed consent. Geneticists have uncovered genes that increase the likelihood of some mental illnesses. Starting in the 1990s, with epigenetic studies — how genes are activated or suppressed in DNA — show that even identical twins can have their genes acting differently.