For your review, a critique of the paper: “1992 Twin Studies of Homosexuality”.
Please scroll down and read: Specific Problems with the Twin Studies of Homosexuality contained in the above link.
Fundamental: In 1953 Sigmund Freud developed a baseline theory of three fundamental causes of homosexuality, all of which must be present for a high probability of homosexuality to occur; they are: 1) A physically frail individual; 2) Any traumatic experiences during childhood; 3) The child’s experiences with the parental situation.
It is Freud’s theory of the three conditions which have undergone individual modification, and not the removal of the triad, from various researchers in recognition that no single condition is the sole cause of homosexuality.
But it is my contention that the practice of cloning possess a theory which refutes that one’s genotype will determine the outcome of personality, character, values, ethics, or psychology, etc, which are the result of upbringing.
A major flaw in the twin studies is that there is such a big difference between Identical Twins (Monozygotic) and Fraternal Twins (Dizygotic) that the findings of twin studies cannot be generalized to the greater non-twin human population as a whole.
Therefore, the findings of any twin studies are only applicable to twins, and not to non-twins.
“Roughly two thirds of identical twins are monochorionic, they share the same chorion, hence the same blood supply, during pre-natal development, one twin receives the mothers blood supply after it has passed through the other twin. The consequences of this on the later development of the twins can include gross phenotypic differences, even before post natal influences of the family are considered. Such a process can lead to an over-estimation of the proportion of variance attributable to the specific post natal environment of the twins.” (Gottesman, 1974) emphasis my own
“Considering the penultimate point, even monozygotic [identical] twins, sharing exactly the same genes, may display phenotypic [observable characteristics] differences due to their different prenatal environment. Things become even more complicated, however, when one considers that not all of an individual’s genes are active at any point in his or her life.”
Dr. Gottesman (1974) stated that "it cannot be over-emphasized tht it is environmental factors through such intracellular metabolic intermediates as hormones, vitimins and toxins that determine which genes get switched on and how long they function … Since only a small portion of the genome (perhaps 5-20%) is activated at any one time, the effective genotype upon which environmental factors are active is constantly changing." Emphasis my own
*3. *Homosexuality is not predetermined in any single gene.
She (Dr. Gottesman) therefore suggested that **“some of the similarity in specific traits is not so much because that trait itself is strongly predetermined, but because the twins were susceptible to environmental influence when they were in similar stages of psychological and maturational organisation. Such factors would lead to an overestimate of heritability estimated from twin studies.” **emphasis my own
“It is, in fact generally found that twin studies of a particular trait suggest higher estimates of heritability than do adoption studies (Plumin 1990). In addition … this may also be explained by nonadditive genetic variance, such as epistasis [where one gene suuppresses the expression of another], which covaries completely for identical twins, but contributes little to the resemblance of first-degree relatives.”
- In Conclusion:
This paper further explained errors in sampling (recruiting enough and proper twins) and stated that “by their very nature (e.g. small sample sizes, bias in recruitments, etc.) they [twin studies] cannot produce results which can be generalized to a wide population.” And “The studies summarized are of inconsistent quality, with biased and limited samples.” The result is that the genetic determination of homosexuality is so far unproved.
Yet , despite cogent examples of sampling error and misinterpretation of evidence, this paper incongruously concluded:
“These results give reason to believe that there is *some *constitutional component to male homosexuality.”
Overall, the **1992 Twin Studies of Homosexuality **appears to offer more counter evidence that twin studies proved the genetic causes of homosexuality, than it does to prove the genetic basis of homosexuality; and actually appeared to be another generalization of Sigmund Freud’s initial thesis.