The local team, from the Queensland Institute of Medical Research and the Queensland Institute of Technology, had questioned 4901 identical twins and non-identical twins and siblings. The study, by Kathy Kirk, Michael Bailey, Michael Dunne and Nick Martin, used a broad definition of homosexuality. Dr Dunne said an earlier study that used narrow definitions of gayness found hardly any genetic influence at all. We found that we didn't find much of a genetic influence. But when we broadened the definition to include things like whether a person had ever been attracted to someone of the same sex, we found a substantial genetic influence.
Human sexuality is very complex and that's true of many aspects of human life.
When you try to work out the genetic influence of these complex human characteristics the extent of the genetic influence is determined by how narrowly you define what you are looking at. Whatever the truth is, it's not going to be straightforward. The team used twins and siblings in the study so they could work out what parts genetics and environment, such as family, played a part in a person's sexuality. Identical twins share the same genetic make-up and the same family environment. Non-identical twins share the same genes as normal siblings but because they were born at the same time, they have the same environmental influence as identical twins.