There have been several studies of identical twins and it is far more common to have one gay twin and one straight twin than to have both twins be gay. From Wikipedia:

http://en.wikipedia.org/wiki/Biology_and_sexual_orientation

A number of twin studies have attempted to compare the relative importance of genetics and environment in the causation of sexual orientation. In a 1991 study, Bailey and Pillard found that 52% of monozygotic (MZ) brothers and 22% of the dizygotic (DZ) twins were concordant for homosexuality.^[4] 'MZ' indicates identical twins with the same sets of genes and 'DZ' indicates fraternal twins where genes are mixed to a similar extent as non-twin siblings. In 2000 Bailey, Dunne and Martin found similar results from a larger sample of 4,901 Australian twins.^[5] Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. They found 20% concordance in the male identical or MZ twins and 24% concordance for the female identical or MZ twins. A meta-study by Hershberger (2001)^[6] compares the results of eight different twin studies: among those, all but two showed MZ twins having much higher concordance of sexual orientation than DZ twins, suggesting a non-negligible genetic component.

Bearman and Bruckman (2002) criticized early studies of concentrating on small, select samples^[3] and non-representative selection of their subjects.^[7] They studied 289 pairs of identical twins (monozygotic or from one fertilized egg) and 495 pairs of fraternal twins (dizygotic or from two fertilized eggs) and found concordance rates for same-sex attraction of only 7.7% for male identical twins and 5.3% for females, a pattern which they say "does not suggest genetic influence independent of social context."^[3]

A 2010 study of all adult twins in Sweden (more than 7,600 twins)^[8] found that same-sex behavior was explained by both heritable factors and individual-specific environmental sources (such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experiences), while influences of shared-environment variables such as familial environment and societal attitudes had a weaker, but significant effect. Women showed a statistically non-significant trend to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twins may influence the results (see below).

Overall, the environment shared by twins (including familial and societal attitudes) explained 0–17% of the choice of sexual partner, genetic factors 18–39% and the unique environment 61–66%. The individual's unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups (other than those shared with a twin), and sexual experiences. In men, genetic effects explained 0.34–0.39 of the variance, the shared environment 0.00, and the individual-specific environment 0.61–0.66 of the variance. Corresponding estimates among women were 0.18–0.19 for genetic factors, 0.16–0.17 for shared environmental, and 0.64–0.66 for unique environmental factors.

So, I don't think genetics are the major cause

Personally I doubt it's anything more than slightly genetic because nature is a relentless bitch who have suppressed genes for far less than making 5% of a certain population explicitly being turned off by the opposite sex and therefore reproduction. Yeah, gay uncle hypothesis and all of that but I'm not sure considering the survival rate to adulthood wouldn't be exactly high among human populations in the wilderness.

I tend to agree with the theories concerning psychological and social factors because people's personalities change with time, sometimes dramatically so, and sexuality could be considered a reasonable part of the human psyche and therefore subject to the same changes. This and predisposition due to hormonal exposure in the uterine environment.

Yes, I should have. I made this thread after some drinking.

sethnintendo said:
Overpopulation

Me: I should have added other. This one made me laugh. So then why don't we see a correlation of gay people with large cities? San Francisco is the Gay capital of USA, but SF isn't that big of a city. Toronto is the Gay capital of North America, but we're only the 5th largest city, and a large portion of our gay people move here because of our pro-gay views and very localized gay neighbourhoods (Church St).

Squilliam said:
Well it can't be genetic in the people who are gay because gay people tend not to breed. My guess is either an environmental factor -> stress, chemicals etc or gender negative adaption I.E. something which helps the mother say produce more children may cause her to produce more gay children.

Me: Well see schizophrenia is cause by a chemical imbalance in the brain. It's believed to be cause by an error in DNA replication (probably a few beliefs, I dunno them all), and it affects the population at random. However you're born Schizo, you don't develop a chemical imbalance. Do you think it's like this?

Gender Negative Adaptation makes no sence, even if the mother could produce more children, but at the cost of more of them being homosexual, that gene wouldn't transmit well between generations because the children are gay. Unless the increase of children is higher then the odds of having a homosexual child. Either way, this is a hormone imbalance, they give women steroids to produce more children.

I think it's either a random genetic trait, or a genetic mutation/defect to inhibit reproduction.

I'm not sure. I think it has to do with the environment people grow up in but then that wouldn't be the case for everyone, cause some people are gay even though they are quite masculine and are into sports and stuff and hang around mostly straight guys, so the environment can't be a factor. hmmm, maybe its some sort of gene.

From what I read it's believed to be because of a variety of factors during pregnancy, not just 1. So if have of the environmental conditions exist in the womb, you're probably still a bit gay, hence this is why there are bisexuals. This would make sense as they are essentially random defects, and thus the percentage of gay people in the population would be pretty random and staggered.

I agree with you about the Bible, a lot of people follow it blindly and really don't understand what's going on in it. You can't follow one section, but not the other. The bible also says women should stay at home and never open their mouths, but christians easily skip over this part. Also the bible never specifically condones Male homosexuality, it condones male prostituion and non martitial sex (but christians won't let gays marry, hence the issue).

The cause for homosexuality is unknown, but the concensus I read is that it's cause biologically. You don't become gay, no tramatic life experience made you gay, and it's not because of some weird sex view that people are gay.

As for who cares? I think in many cultures Gay people are really oppressed. It's not so bad here in Canada, we can talk about stuff like this without creating too much hate (depends on who or where you talk about this). So I think a lot of people who ask this are just sick of seeing gay people opressed.

I'm interested in it because I think of homosexuality and a non harmful defect, but there should be some effort to look for a cure. I want to know how many people think it's a defect too. If it's cause by social environment, then there probably isn't a full proof cure, but if it's because of an error in pregnancy (like too much Vitamin D, or something minor) then it would be good to know so that parents can do what they can to help reduce the odds of having gay children, and thus grandchildren. I think homosexuals should have the option to remain homosexual, or attempt to become heterosexual, and the drugs that are available now to do this have a lot of negative side effects, mainly a low sex drive lol.

There's been a thread on this if you want to read it.

I personally lean towards the psychological explanation. Inherent traits tend to have very little to do with any preferences and desires. A very large chunk of a person's behavior and mental processes are the result of what they've learned or experienced. I don't know exactly what factors cause homosexuality, nor do I know when after a person's birth are a person's sexual desires even formed. I can't say for sure there are no genetic factors though, so I won't dismiss it entirely. Sexual preference is 85-90% psychological and 10-15% biological imo.

What a person learned from this reinforcement and punishment could remain in a person's subconscious for his entire life potentially deciding who he is attracted to and what he percieves as good or bad. There probably aren't any definitive evinronmental factors that for sure causes certain sexual preference though. Considering how complicated and bizarre the human mind is, I'm pretty sure the cause of sexual preference is also complicated and perhaps bizarre. I think sexual preference is caused similar to most preferences of our mind; that is, we learn behavior and form beliefs, personalities, values, etc. as a result of what we learn in life.

This does not mean that I think homosexuality is a choice though, at least not a conscious choice. Homosexuality is no more of a choice than a person's favorite color or favorite music genre or favorite hair color imo. Obviously, people aren't born with these preferences, thus the causes of those are mainly psychological, but that doesn't mean it's a conscious choice. I would say that it's a subconscious 'choice.' Essentially, a variety of factors combine and collide inside a person's mind and it eventually makes them desire certain things over another. 

I personally don't think it's biological, at least not for most people. People don't even have an understanding of gender until after birth, much less any attractions to any specific genders. Likewise, I don't see how people would already have sexual preference before they can even understand the difference in genders. How does that work?

we love lesbians. well i do anyway, and bi women.

i voted hormonal imbalance, but it could just be the way things are. could also prove there's no god, or god didn't create humans.

What about random mutations? A lot of disorders are cause by random mutations which affect the population as a whole, and have nothing to do with inherit traits.

Also children raised by same sex and opposite gender parents have identical chances of becoming heterosexuals. If environment made a difference, how come it doesn't here?

As for preferences, a few people came up with this. However, twins separated at birth, raised in different environments typically share similar taste in spouces. They usually marry people with a similar background, similar personalities, and even similar names. Why then can there exist 1 homo and 1 heterosexual in identical twins? It's more common in fraternal twins, but even in identical twins, who are raised in the same household, one can be gay, and the other not.

you mean genetic/genes! there could be a mutation within genes, or the brain, and then there are humans that experiment, but one things for sure.

it's not about choice. you can't help who you fall in love with. (love is a waste of time and energy) i need to stop watching Jerry springer.

kids being raised by same sex or opossing gender couples have the same chance of being straight or gay.

in the end there are to many unknown variables to draw any real conclusions.