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. And normal siblings are the same distance from each other as two non-identical twins but their family environments would be slightly different because they were born at different times.