Professor Chong Siow Ann wrote about the removal of homosexuality as a disorder by the American Psychiatric Association in 1973 and quoted the study conducted by neuroscientist Simon LeVay on brain abnormality in homosexual men to support his argument that biological factors play an important role in the development of sexual orientation ("Coming out: How parents react is crucial"; Nov 21 ).
However, the weakness of the LeVay study (Science 1991) is well known.
The study reported that the size of the third sub-group of the anterior hypothalamus nuclei in 19 heterosexual men were two times larger than those in 13 presumed homosexual men who died of HIV.
The study has never been replicated.
Even if the difference in size is truly statistically significant, the association does not mean causation.
The difference in size could be caused by HIV infection rather than the biological cause of homosexuality.
Furthermore, the boundary of the third nuclei is very indistinct, making precise measurements difficult.
To date, homosexuality is not known to be a chromosomal anomaly, like in Down syndrome, nor a Mendelian disorder, such as haemophilia.
Even the much-quoted Xq28 chromosomal linkage study (Hamer et al. Science 1993) has not been substantiated by a larger study (Rice et al. Science 1999).
Studies of twins by concordance rate and heritability have revealed that environmental factors are much more important than genetic influence. Research in sexual orientation also shows plasticity which is influenced by many social factors.
American psychiatrist Robert Spitzer, who championed the delisting of homosexuality in 1973, reported 30 years later lasting change in sexual orientation after re-orientation therapy (Archives of Sexual Behaviour 2003).
All these argue against Prof Chong's conclusion that homosexuality is an inborn characteristic which is involuntary in people.
William Yip Chin Ling (Professor)
PROFESSOR CHONG SIOW ANN'S REPLY:
Professor William Yip Chin Ling has, unfortunately, taken a rather selective sampling of the extant and extensive published scientific literature to assert his view of the efficacy of re-orientation therapy.
He has given the view of a single psychiatrist, Robert Spitzer, who in 2001 had stated in a paper that it is possible that "highly motivated" individuals could successfully change from being homosexual to heterosexual.
Prof Yip has omitted to mention that the American Psychiatric Association then issued an official disavowal of this paper - stating that "there is no published scientific evidence supporting the efficacy of reparative therapy as a treatment to change one's sexual orientation".
Although the paper was subsequently published in 2003 in the Archives of Sexual Behaviour, it was heavily criticised for the flaws in the study methodology.
There is a coda which Prof Yip is, perhaps, not aware of.
In an interview in 2012, Spitzer told the reporter of the magazine, The American Prospect, that he had asked for his paper to be retracted - admitting that he had by then agreed with his critics.
In his letter to the editor of the Archives of Sexual Behaviour to request the retraction of the article, he wrote: "I believe I owe the gay community an apology for my study making unproven claims of the efficacy of reparative therapy. I also apologise to any gay person who wasted time and energy undergoing some form of reparative therapy because they believed I had proven that reparative therapy works with some 'highly motivated' individuals."