Is Homosexuality Genetic?
Twin Studies: Is Homosexuality Genetic?
Written By: Dr. Neil Whitehead ( http://www.mygenes.co.nz/ )
(Posted January 2008)
The answer to all the alleged biological influences is conclusions from - Twin Studies.
In this paper I describe a very simple form, which avoids many complications and gives us the information we need. Usually identical twins are compared with non-identical twins. In the case of SSA this has given very ambiguous results, but we find if we only consider identical twins, the situation becomes much clearer.
We compare whether both identical twins have a given trait. Identical twins have identical genes. And they usually have identical womb environment. And they have virtually identical upbringing. So if they both have the same trait, probably some combination of common genes, prenatal factors and upbringing is responsible. So the critical question is if one twin has SSA, does the other twin? And in what percentage of cases?
Look carefully at the next figure:
Figure 8. Pairwise concordance among identical twins for 1 Lung Cancer, 2 Criminality, 3 Stroke, 4 Breast Cancer, 5 Same Sex Attraction, 6 Leukemia, 7 Malformation, 8 Alzheimer's, 9 Ulcerative Colitis, 10 Rheumatoid Arthritis, 11 Alcoholism, 12 Schizophrenia, 13 Depression, 14 Suicide attempt, 15 Diabetes type I, 16 Divorce, 17 Crohn's disease, 18 Asthma, 19 Hypertension, 20 Co twin is best friend, 21 Diabetes type II, 22 Autism, 23 Opposite Sex Attraction, 24 Phenylketonuria
The top point, #24 is phenylketonuria, caused by a known gene defect. If one twin has it, the other twin almost always has it. #23 is OSA. The concordance is very high. If one twin is OSA the other is usually OSA as well. However we cannot tell from this whether it is caused by genes, upbringing, or some combination. The concordances decrease as we move to the left, until we reach SSA which has a concordance of 11%. The other points with that kind of value are the cancers, which all oncologists will tell you have a large element of chance involved. SSA has a large element of chance in it. In fact we have to say it has an unusually large element of chance in it compared to most traits. By change we mean non-shared events, which affect one twin and not the other.
This 11% is a terribly important statistic. All the common biological and social influences known and unknown and yet to be discovered, add up to 11% concordance for men and 14% for women twins. All that long list of biological influences previously given in this paper are included, and they only add up to 11-14%.
Four papers agree that SSA pairwise concordance has about this value. One of them, a paper on adolescent twins found an SSA concordance of 7% for men and 5% for women, which is even lower but in the same ballpark. So we have to take it the figure is accurate and not going to change. If one twin has SSA the other usually does not.
There is one complication. People may say to you "I thought the concordance figure was 30%" or they may say "52%". It is a fact that the early studies had higher figures but are also now universally acknowledged (and by their authors) to be highly biased samples. It is also a fact that they were quoting a concordance called "probandwise concordance" which is more complex than the "pairwise" concordance I quoted to you, significantly higher, and used in later calculations that do not immediately concern us in this paper. The only important conclusion is that whichever form is used, SSA would still be clustered among the traits with a very high chance component. The irony is that far from being a trait with extremely high degree of dictation by genes, it is almost as far from that as it is possible to get. This is another sad example of the public tending to believe the exact opposite of the truth in scientific matters
SSA dictated? The exact opposite. I hope that from now on you will not be disturbed into thinking the genes or prenatal conditions are overwhelmingly important, regardless of what new discoveries may emerge.
Four years ago I spoke at the annual NARTH conference, and used the higher probandwise concordance figures. At that stage I was too conservative. I said the genetic effect was small. Now I would say "unusually small".
The gay activist may say to you - "Oh SSA is still dictated by the genes, but they are not exerting their effects- this is a known genetic phenomenon called poor penetrance". The answer is that you have to have a known gene or genes producing the trait before you can say that, and in this case there are no genes. You might say to such a person "Please come back when a gene is found!"
Now I want to clarify some points because there is significant risk of misunderstanding. I am not saying 11% of all twins have SSA - only about 2-3% do. I am not saying when a twin has SSA that in their case there is genetic influence and in other cases no genetic influences - all of us are subjected to all the genetic and social influences.
Why twins might differ
Now this analysis I am presenting shows that chance - non-shared experience - is an unusually large factor and accounts for differences between identical twins. What would be examples of this?
- One twin sees internet SSA porn and the other doesn't
- One twin misperceives the father favoring the co-twin
- One twin is unlucky in (heterosexual) love and thinks he is gay
- One twin is sexually abused and the other is not
Chance events, non-shared circumstances can lead to very different endpoints.
Now a disturbing factor for some at this conference may be to hear that not only are genetics a minor factor but family circumstances are also. While this is generally true for the vast majority, there is a small minority for whom the family circumstances are vitally important. But if one twin rebels against the masculinity expressed by his father or peers, the other usually does not. This is a chance occurrence, and as a generalisation parents are mostly not to blame for the SSA of their children. Accidental misperception of parental motives is much more common.