Tuesday, May 09, 2006

Something Smelly

A new study reports on the reactions of volunteers' brains to male and female pheromones. Some of the volunteers were heterosexual men and women, some were lesbians. The same researchers conducted an earlier study comparing heterosexuals to homosexual men:

The research team led by Ivanka Savic at the Stockholm Brain Institute had volunteers sniff chemicals derived from male and female sex hormones. These chemicals are thought to be pheromones — molecules known to trigger responses such as defense and sex in many animals.

Whether humans respond to pheromones has been debated, although in 2000 American researchers reported finding a gene that they believe directs a human pheromone receptor in the nose.

The same team reported last year on a comparison of the response of male homosexuals to heterosexual men and women. They found that the brains of gay men reacted more like those of women than of straight men.

The new study shows a similar, but weaker, relationship between the response of lesbians and straight men.

Heterosexual women found the male and female pheromones about equally pleasant, while straight men and lesbians liked the female pheromone more than the male one. Men and lesbians also found the male hormone more irritating than the female one, while straight women were more likely to be irritated by the female hormone than the male one.

All three groups rated the male hormone more familiar than the female one. Straight women found both hormones about equal in intensity, while lesbians and straight men found the male hormone more intense than the female one.

The brains of all three groups were scanned when sniffing male and female hormones and a set of four ordinary odors. Ordinary odors were processed in the brain circuits associated with smell in all the volunteers.

In heterosexual males the male hormone was processed in the scent area but the female hormone was processed in the hypothalamus, which is related to sexual stimulation. In straight women the sexual area of the brain responded to the male hormone while the female hormone was perceived by the scent area.

In lesbians, both male and female hormones were processed the same, in the basic odor processing circuits, Savic and her team reported.

Each of the three groups of subjects included 12 healthy, unmedicated, right-handed and HIV-negative individuals.

Thirty-six individuals... There are a lot of good reasons to be concerned with such a small sample size, unless all the between-group differences had hundred percent prevalences. Think about how much power a sample of this size can have on public debates. I find it a little scary.

But I'm more concerned with this reaction to the study:

In both cases the findings add weight to the idea that homosexuality has a physical basis and is not learned behavior.

"It shows sexual orientation may very well have a different basis between men and women ... this is not just a mirror image situation," said Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Hamilton, Ontario.

"The important thing is to be open to the likely situation that there are biological factors that contribute to sexual orientation," added Witelson, who was not part of the research team.

What does it mean to say that "it shows...homosexuality has a physical basis and is not learned behavior"?* Are learned behaviors somehow causing no brain reactions? I doubt that. More generally, that something shows up in the brain does not tell us that it always showed up in the brain the same way. Experiences we have (such as depressive illnesses) can change the way the brain reacts. What if having sex with a certain sex changes the way your brain reacts? Note that I'm not arguing against homosexuality having a physical basis. I'm arguing against the increasingly common assumption that brain scan differences are proof for a genetic explanation of behavior. Think about people who are bilingual. Their brains scan differently than the brains of monolinguals but the second language is certainly learned.

This seems like a good place to tell my Interesting Smell Story, even if it has only a peripheral link to the main topic of the post:

Before I became a vegetarian fried chicken smelled like fried chicken to me, hot dogs smelled like hot dogs, meatballs like meatballs and so on. I could identify the foods that were cooking by their smells and all those different smells said "food" to me. If I was hungry I'd inhale the smells with deep enjoyment.

Some years after I stopped eating meat I realized that I had lost the ability to identify the smell of frying chicken or meatballs or hot dogs. I literally can't smell them anymore. All I smell is something that burns and releases nasty fumes.

I have tested this and my ability to use smell to guess what meat is cooking has gone. All that remains is a general identification of burning flesh and a slightly unpleasant reaction to it. (I still identify smells such as garlic and mushrooms and react to those smells with pleasure if I'm hungry.)

At the same time, I have a very clear memory of the smell of fried chicken or meatballs. Now why can't I actually identify those smells in reality when I can recollect the smell quite well?

You get the point of the story by now.
*As JR pointed out in the comments, I manufactured this quote. My apologies for doing it, even if the reason was fatigue. Replace the word "shows" with the words "adds weight to the idea".