we fairly acknowledge that the following text is an excerpt  from:
Clarke, V., Ellis, J. S., Peel, E., & Riggs, D. (2010). Lesbian, Gay, Bisexual, Trans and Queer Psychology An Introduction. Cambridge: Cambridge University Press. ISBN-13 978-0-521-87666-7 (pages 3 and 4).

LGBTQ Psychology

Why do we need a separate branch of psychology for LGBTQ (and/or non-heterosexual) people? There are two main reasons for this:

  1. Until relatively recently most psychologists (and professionals in related disciplines such as psychiatry) supported the view that homosexuality was a mental illness. ‘Gay affirmative’ psychology, as this area was first known in the 1970s, developed to challenge this perspective and show that homosexuals are psychologically healthy, ‘normal’ individuals.

2. - Second, and related to the pathologisation of homosexuality, most psychological research has focused on the lives and experiences of heterosexual and non-trans people. LGBTQ people are given little or no consideration within mainstream psychology. For example, most research on mothers is based on heterosexual mothers, and prejudice against LGBTQ people is given scant attention in social psychological research on prejudice. LGBTQ psychologists believe that if psychology is to be a true ‘psychology of people’, then it must examine the experiences of all people and be open to the ways in which people’s lives differ. 

Without comprehending the lives of both LGBTQ and non-LGBTQ people, no psychology can claim to be comprehensive.

Literatura:

Clarke, V., Ellis, J. S., Peel, E., & Riggs, D. (2010). Lesbian, Gay, Bisexual, Trans and Queer Psychology An Introduction. Cambridge: Cambridge University Press. ISBN-13 978-0-521-87666-7.