Twin studies, conducted by professionals, help to determine the genetic and environmental impact on identifiable conditions.
As identical twins have exactly the same pre-birth environment and genetic predisposition, it is safe to assume that any differences between the twins is caused by their subsequent changes in environment or habits (for mature twins). Fraternal twins have the same environment including the same pre-birth environment but do not have the same genetic endowment.
Some twin studies are clear cut: identical twins both have some conditions but one fraternal twin can get the condition while the other does not or both fraternal twins get the condition (indicating environmental factors) and so on. The Twin study is a tool that can be used to probe the nature-nurture component of some conditions.
Like most tools, it should be used in conjunction with other tools and usually is. In the case of schizophrenia, no clear correlation between identical twins is seen but the figure is high enough (50%) to show that there is a genetic factor which is not prominent enough on its own to cause the condition.
The likely cause of schizophrenia, then, is the combination of environmental and genetic factors. The 17% for fraternal twins is way higher than one would expect for a genetic only condition, and the condition does run in families but not in any definite way (a bit like psoriasis - though thought to be genetic in nature, non of my parents or grandparents or my cousins have the condition beyond the regular elbows and knees). As with manic depression, the majority of sufferers have no relatives with the condition.
What confuses many people is the commercialisation of identical twin studies done in the USA by the Minnesota University. Here is a snip from a book that reveals some of the flaws in these studies [1]:- "Leon Kamin, a psychologist at Northeastern University in Boston, has contended that the Minnesota twins have various motives to downplay previous contacts and exaggerate their similarities. The twins might want to please the researchers, attract more attention from the media, or even make money. There was evidence to support Kamin's suspicions. Whereas some news accounts suggested that Oskar and Jack (the Nazi and Jew) and the two giggle sisters had been reunited for the first time in Minnesota, both pairs had met previously. James Springer and James Lewis, after being discovered by Thomas Bouchard, appeared on 'Johnny Carson Show' and in 'People' magazine. Some twins acquired agents and Jack, after being profiled in a long article in the 'Washington Post', sold their life story to a film producer in Los Angeles. (The film was never made.)
Questions have also been raised about the motives of the Minnesota researchers. The publicity generated in the 1980s by the stories about Oskar and Jack, the two Jameses, and the giggle sisters led to a surge in funding for the Minnesota team. Its most generous backer has been the Pioneer Fund, a remnant of the American eugenics movement. The organization's name refers to its original goal of promoting the propaganda of descendants of America's "pioneers" - that is, white Anglo-Saxons. The Pioneer Fund has supported various groups and scientists who advocate racial segregation.[2]
Most of the Minnesota researchers have disavowed eugenicist policies. One exception is the psychologist David Lykken, who has proposed that the government raise the IQ of its citizenry by allowing only women who meet certain criteria to bear children; "unlicensed" women who become pregnant would be forcibly implanted with the birth-control drug Norplant. Lykken complained to the reporter Lawrence Wright, "A of lot social scientists are so scandalized by my proposals that they think I must be a fascist." A caustic reviewer of 'Twins' remarked, "Now, why would they think that?" [1]
One of the main claims of the Minnesota group is to have examined numerous identical twins that were separated at birth. Psychologist Susan Farber examined 121 pairs of twins separated at birth and found only three cases where the twins had never met before the study. It seems that all that publicity and money has corrupted the Minnesota results far to much for scientific use. Would unlike (in habits) twins be featured on TV? Why did the university offer money to entice twins into their study? Wasn't the television coverage explaining to twins just how they should present if they wanted to become (slightly) famous?
In sum: ignore the commercially done twin studies - far too corrupted. However those twin studies done by sober scientists are of value.
[1] The Undiscovered MIND: How the Human Brain defies Replication, Medication and Explanation by John Horgan (I highly recommend this book).
[2] in other words, if we are beholden to our genes then no amount of education will improve us, at least this is the implication. If twins really were identical then genetically similar people can also be lumped together and treated as a block eg various non-white people.
© Robert Karl Stonjek 2002