Summarize what research has taught us about sexual orientation and its development

Sexual orientation is not an indicator of mental health.

There is no evidence that environmental influences determine sexual orientation.

Evidence for biological influences includes

the presence of same-sex attraction;

straight-gay differences in body and brain characteristics;

higher rates in certain families and in identical twins;

exposure to certain hormones during critical periods of prenatal development;

and the fraternal birth-order effect.

cite: www.uucnc.org