Bryan Caplan  

Nature, Nurture, and Orientation

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Born Gay?... Ralph Nader and Campaign Finan...
My take from Selfish Reasons to Have More Kids:

Parents have a small effect on sexual orientation. Psychologists used to label homosexuality a "mental illness" caused by overprotective mothers and distant fathers.[i]  Now we tend to see sexual orientation as a preference inherent in our genes.  When you look at the actual evidence from twin and adoption studies, however, neither story quite works.  Genes definitely play a strong role - every major twin study finds that identical twins are more alike in their sexual orientation than fraternal twins.[ii]  Yet genes are far from the whole story - if you're gay, your identical twin is usually still straight.  Furthermore, family environment clearly affects sexual orientation.  The smoking gun: Adoptive brothers of gay men and adoptive sisters of gay women are about six times more likely to be gay that you would expect from chance.[iii]  On balance, the evidence that parents make kids gay is a little stronger than evidence that parents make kids well-educated, rich, pious, or chaste.  While heredity is the star of the story, upbringing plays a modest supporting role.

[i] Bayer, Ronald.  1981.  Homosexuality and American Psychiatry: The Politics of Diagnosis.  NY: Basic Books.

[ii] "Genetic and Environmental Influences on Sexual Orientation," in Handbook of Behavior Genetics, p.271.

[iii] "Genetic and Environmental Influences on Sexual Orientation," in Handbook of Behavior Genetics, p.271.



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COMMENTS (10 to date)
chipotle writes:

Some men live to a ripe old age and never write a blog post this awesome.

Does Professor Caplan or anyone else care to speculate what sort of environment makes homosexual orientation a more likely outcome?

Jody writes:

I've grown fond of the chimera hypothesis though I've not seen it tested.

Implies homosexuality is innate, but not genetic in the sense we normally mean, and at the same time both genetic and environmental in cause.

agnostic writes:

I suggest the behavior genetics term "shared environment" be used instead of "nurture," "upbringing," etc. We don't know that an effect of sharing an environment means that it was a particular aspect of it such as parenting style, so that's too strong of a claim. Steven Pinker does a good job of emphasizing this point in The Blank Slate.

If there were variation the levels of radioactive waste beneath people's homes in a population, you would find a shared environment effect on IQ, height, etc., due to the kids in one home being more greatly harmed by waste than those in another, whether these kids were genetically related or not.

Someone in the previous thread mentioned Greg Cochran's "gay germ" thesis, which has made the most sense to me. Exposure to pathogens varies across households, so that's the shared environment effect. Individuals may have genetic differences that make one person more and another person less susceptible to being infected (or to progressing to homosexuality upon infection), so that's the heritable effect.

And of course chance plays a role in all infections -- the bug zigs instead of zags, and this means one person gets a worse outcome than another. For example, polio likes to live in your gut; only by chance does it get lost and find its way into the nervous system.

Peter Twieg writes:

Uncharitable summary of Bryan's book: "You can't make your kids smart, but you can make them gay!"

RAD writes:

So anything that is not due to heredity we can chalk up to upbringing?

Is there something in the adoptive sibling study/studies that helps identify familial vs. peer influence?

twv writes:

The most promising theory of the origin of male homosexuality stresses the adapting hormonal make-up of the womb, under successive pregnancies. It explains one of the odder and most astounding statistical facts of homosexuality: The more older brothers a man has, the more likely he is to be gay.

Basically, the theory says that women's bodies learn to suppress testosterone in male prenates, over time. Their bodies learn how to suppress unwanted hormone production.

Hormones seem to be a major part of the puzzle. Hormone production is key, especially the production in the womb during a certain critical time.

This being said, based on my conversations with many, many heterosexual, homosexual, and bisexual males, I hazard that some people do engage in homosexual acts simply because of learned pleasure. The "try anything once" attitude encourages experimentation, and the experience of sexual activity tends to be pleasurable, no matter what the partner.

Certain experiences of a negative sexual nature can influence a later turn to homosexuality. I've not seen studies, but I have come across a great deal of anecdote. It seems likely.

The idea that there is a single cause for a behavior or even a behavior pattern seems unlikely to me.

I don't think there's a single cause for some of the deepest perversities, such as voting Democrat or Republican.

Corey writes:

Could it be possible that adopted children with gay siblings grow up in an environment where it's more acceptable to admit homosexual tendencies and therefore are equally likely as a random individual to be homosexual, but more likely to admit it on a survey?

Lord writes:

Yes, perhaps it is not you can make them gay but you can repress them.

carl writes:

The older brother theory is challenged/debunked by Andrew Francis of Emory Univeristy in this study:


http://www.informaworld.com/smpp/content~content=a904517754~db=all~order=page

I do agree with a couple of previous posters that Greg Cochran's idea makes the most sense and it's probably the explanation most gays least want to investigate or hear about, yet the one most prospective parents would be most eager to see tested.

In addition to reading Cochran, you might read some of Paul Ewald's stuff on infectious causation of disease. It's not at all ususual for certain pathogens to be passed back and forth among members of a family, equally not unusual that family units have biological/genetic susceptibilities to particular pathogens. This could explain the phenomenon of some degree of family clustering noted by Bailey and others.

Allison Bracken writes:

I think that being homosexual is a combination of nature and nurture. Some men are born very feminine, and some women very masculine. However, this does not interfere with the sex that they are attracted to all the time.
If a person who already has these qualities was raised poorly, or if their parents were overprotective, they could become gay.
However, this is not the only factor. For example if a woman was in an abusive relationship with a man, they may begin to date women because they are comfortable with them. Therefore, it is a combination of nature, nurture, and past relationships.

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