rat sex.

Narrator: But first, he told me I would need a crash course in rat sex.

I'm watching a video for psychology. This week's chapter is on sexual motivation, and the aforementioned video is entitled "Homosexuality and the Nature-Nurture Debate."
The answer is pretty conclusively nature, by the way, even though identical twins can be different things.

Narrator: So does the fact that [twin] Steve is gay and [twin] Greg is straight prove that it's not in the genes?
Scientist: Well it proves that sexuality isn't entirely genetic.
Narrator: (pleased with herself) Well that brings us back to the mother and father!
Scientist: No it doesn't.
Narrator: Yes it does! That's the environment!
Scientist: No. We're talking about the environment in utero.

The narrator has an agenda, clearly. She's some trashy blonde aging woman in denial, in the late eighties (from what I can tell).

Anyway, one of the ways they know that these things are mostly influenced by biological factors is that they can manipulate rats' sexual orientations. And that's what brings us to the quote at the top.

[Edited because this narrator is definitely high]

Scientist: The more older brothers a man has, the more likely he is to be gay.
Narrator: Say that again!
Scientist: The more older brothers a man has, the more likely that man is to be gay.
Narrator: Is that true!?
Scientist: That is absolutely true.
Narrator: Absolutely true?!?!
Scientist: Absolutely.
Narrator: ..... Wait a minute....

Also, here's a funfact. One of the sexual-orientation scientists' names is Dr. Breedlove.

Love always,
Clara