To test whether homosexual behaviour really does go hand in hand with a need to form close bonds with others of the same sex, they decided to compare sexual behaviour with the level of a certain hormone in the blood.
Progesterone is also known as a “social” hormone – it is produced mainly in the ovaries in women and in the adrenal glands in men, and encourages a person to engage with others in a positive way.
They asked a group of participants questions such as "The idea of kissing a person of the same sex is sexually arousing to me," and: "If someone of the same sex made a pass at me I would be disgusted”. They compared the answers to the level of progesterone in the blood.
The results, published in the journal Archives of Sexual Behaviour, show that the people with higher levels of the hormone are indeed more inclined to have homoerotic thoughts.
A glance at the progesterone molecule itself gives a bit of a clue.