The last 20 years have seen an explosion in the scientific knowledge available on the genetic, biological and psychological causes of homosexuality. Scientists are now in agreement that homosexuality is not a freely made choice; it is not a "lifestyle" that someone decided to follow. Instead, it has been established with a high degree of certainty that there are genetic causes which make people more likely to become homosexual.
It is unknown how many types of genetic variations lead to homosexuality. It is also unknown to what extend one's gene's compel the development of homosexuality. Many other biological factors are likely involved. It is also unknown for certain how much social and psychological influences can affect a person's sexual orientation. The following topics need to be elucidated in this entry, with references to imporant scientific and medical journal articles, and books which summarize said articles.
Genetic studies
Twin Studies
Biological Studies not strictly dealing with genetics
Interplay of the environment and the biological make-up of the individual. (Nature vs. nuture)
The world's various religions have differing views on homosexuality; this topic has its own entry entitled Religion_and_homosexuality.
This entry is only about causal theories. It is not for discussing issues of morality, ethics, human rights, religion, etc. This is not a place to make a point about supposed "sins."