My favorite ones are those where women sniff the shirts of various men and pick out the ones that smell the best to them. The ones they like best are usually from men whose immune systems are most different from theirs.

This makes some sense if you think about it. The strongest immune system is a varied one. It can fight off lots of different kinds of bacteria, parasites, and viruses.

And since our immune system is programmed by our genes and our genes come from mom and dad, the more different mom and dad’s genes are, the more varied your immune system will be. So ideally you would choose a mate that would give your children the stronger immune system. That mate would have an immune system very different from yours.

One way you might be able to find that particular mate is if different immune systems have different odors. The stinky guys share your immune system; the pleasant smelling ones have a different one.

This seems to be what is happening in these armpit smelling experiments. So at least some amount of attraction between people is happening with the nose.

But what I found even more interesting was that oral contraceptives mess with this system. Women on the pill tend to prefer the smell of men with similar immune systems. Some scientists think this is because pregnant women prefer family around them and family tends to have a similar immune system.

When I first read this I thought, “Wow, this is going to really mess up future generations’ immune systems. Maybe it even helps to explain the recent rise in allergies and autoimmune diseases.”

But then I caught myself and thought a little harder. It is probably not that likely that the recent increases in allergies and autoimmune diseases would have happened so quickly if the pill were the main culprit. And besides, there is undoubtedly more to human mate selection than smell!

We are complicated. There are all sorts of cues that cause someone to fall in love. Pheromones may play a role but they are certainly not the whole story.

For example, some studies use photos and ask women which men are the most attractive. The women in these studies tend to pick men with more similar immune systems (apparently there is some correlation between facial symmetry and the immune system).

And when scientists look at couples who actually have children together, they get mixed results. An early study pointed to Europeans having children with partners with different immune system genes. But a deeper look at that data and looking at a larger group of couples showed that this didn’t seem to be the case. Differing immune systems had little impact on who women chose to have children with.

The most that could be said was that women tended to avoid men with really similar immune systems. A bit similar was OK.

Of course we could already be seeing the effects of the pill in these studies. Maybe if the women in these studies hadn’t been on the pill when they met their partners, they might have chosen someone with a more dissimilar immune system.

Scientists need to look at people who did not meet while women were on the pill. Then we will have a better idea of how big a role your nose plays in choosing your soul mate. And how worried we should be about the pill’s effect on that selection.

Nice PBS video about the t-shirt smelling experiments.

Last night, I watched a reality dating show with a seemingly wacky way of finding true love. The male searching for love sniffed the armpits of potential females. He either turned away in disgust or became quite aroused by the wafts of underarm aroma. What is so comical is that a new dating service relies on a more sterile but ultimately similar approach. Scientific Match, which opened last December, provides dating services based on both personality and DNA compatibility. Clients send in a cheek swab of DNA to be matched up with a potential mate who has dissimilar DNA.

Attraction based on dissimilar DNA was first studied in 1995 by Claus Wedekind. The study is referred to as the "Sweaty T-Shirt Experiment." In the study, women were given t-shirts recently worn by men. The men in the study were not allowed to wear any cologne or scent that would mask their natural scent. The women were then asked to rate how "sexy" the t-shirts smelled to them. Correlation was found between how good the shirts smelled to the women and how dissimilar their immune DNA was to the man in question. Following this approach, Scientific Match matches clients based on chemistry, specifically making sure six immune genes of each male and female do not match.

But why would it be advantageous to be attracted to someone with dissimilar DNA? Scientifically speaking, it increases the robustness of the species by providing genetic diversity in a gene pool. Diversity enables survival in intense bouts of environmental and infectious selection. There is more of a chance that an individual will have the adaptation to survive and propagate in the population and pass on the advantageous trait. Similar DNA creates a bottleneck in the gene pool because similar genes are passed down rather than diversified – this is the case with inbreeding.

However, basing attraction on chemistry has its drawbacks. Women who are on birth control are turned away from Scientific Match because findings have reported that they are attracted to men with similar immune system genes. Birth controls works by tricking the body in believing it is pregnant. Studies have shown this also changes the indication of genes making women more attracted to family members than potential mates. This behavior has been seen in other mammals and is thought to be a way of protecting a pregnant member of the family and its offspring from harm.

If you meet the qualifications for Scientific Match, you might just find the love of your life for a whopping price tag of a $1,995. That or you can try the more wacky approach of smelling someone's armpits! Either way, companies such as these denote advances in technology. DNA sequencing and comparative genomics have become cutting edge science and they are infiltrating our everyday life. Genetic testing is not only in the dating arena. Swab and send testing kits are being used for other projects such as National Geographic's Genographic Project and 23andMe. Both use DNA to uncover deep roots of family trees by using comparative DNA.

Cat Aboudara is the Special Projects Manager at California Academy of Sciences and works in the public programs division. The Academy is a wonderful fit for her because of her curiosity about the natural world and her experience in working with native California wildlife.

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