Strong chance factors in SSA
N.E.Whitehead, Ph.D
We can tell how much chance* is involved in the development of SSA (Same Sex Attraction) or OSA (Opposite Sex Attraction), or other traits, by looking at twin studies.
We take a large set of identical twins 1, and find the small number of pairs in which at least one twin has SSA. Of those pairs, the percentage in which both twins have the trait is called the “ pairwise concordance”. For SSA the pairwise concordance is shown at position 5 in the graph below, along with pairwise concordances for 23 other conditions.
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 pairwise concordance results from all the factors which the twins have in common. These are genetic factors (because identical twins are identical genetically), upbringing (family), and other social influences). The data for the graph are all from scientific journals - references on request. For phenylketonuria (a genetic/enzyme condition ultimately causing mental retardation) the concordance is 100% (without medical intervention). For OSA some combination of genetics and other common social environments gives a high concordance, but we cannot tell the relative proportions without studying fraternal twins as well.
If the concordance is poor, it means that the combination of genetics and upbringing has little influence and erratic random factors are very important, and this is true for SSA and the other traits on the left-hand side of the graph. Also in this category are most cancers, stroke, and criminality. These are all dominated by random circumstances in life and/or an individual's personal reactions to them.
This means that OSA and SSA contrast considerably, and the old argument over whether SSA is predominantly nature or nurture has the answer “Neither – it is mostly chance”, which will probably please no-one! That is, SSA is a highly individualistic response to events and situations in life, while those who develop OSA are going along somewhat passively with the same raw material.
(1). Here we use only the most primitive comparison, comparison of identical twins, because the unusual mathematical form of SSA and OSA distribution makes the more usual complex twin studies difficult and gives them very large errors. This problem is discussed in Bailey et al (2000) from which SSA and OSA data are drawn for the current graph. Other recent studies support the data.
Reference:
Bailey JM, Dunne, MP, Martin, NG (2000) Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Journal of Personality and Social Psychology 78, 524-536.
*Chance: individual deep-seated reaction to a random event or events experienced as traumatic, that reaction so deep as to affect gender development by being repeated and becoming habitual.