The Science of Attraction

“When you see couples around campus [you wonder] what do those people see in each other? They don’t have anything in common,” said Zehra Hussain 09. Hussain, along with Helen Chmura ’09, Aleta Hong ‘09, and Melinda Yang ’10 from two biology seminars conducted a t-shirt smelling experiment, this past weekend, to investigate the biology of romance.

Hussain initially discovered the topic while researching for a thesis project, over winter break. “I think someone mentioned it to me, and I thought it was a really cool idea,” she said. Yang, from the Molecular Ecology and Evolution biology seminar, contributed the molecular biology part of the hypothesis. The professor of her course, Vincent Formica, suggested that Hussain’s project idea would relate to Yang’s interest in human evolution. When Hussain suggested the project topic to her class, several people became really interested.

One part of the hypothesis suggests that people are attracted to people with higher hormone levels, testosterone and cortisol, for evolutionary reasons. Studies show that testosterone is related to high social dominance, which makes such individuals more likely to succeed evolutionarily. Similarly, studies theorize that high cortisol levels are correlated with strong immune systems, making individuals more evolutionarily “fit.”

“Mate selection is based on evolution, passing on genes to create most “fit” offspring,” explained Hussain. “By the hormonal basis, it suggests that hormones indicate the fitness level of the male.”

Studies have shown that females preferred the scent of males with high levels of testosterone, although odor is gender-neutral. Thus, Hussain tested the hypothesis that people were more attracted to people who had high levels of these hormones.

Yang tested the hypothesis that a couple with dissimilar genes in the major histocompatability complex (MHC) is more attracted to each other. MHC is a gene area in chromosome six that codes for proteins relating to the immune system. These proteins make foreign peptides that help T-cells recognize foreign pathogens and destroy them if they are harmful to the body. The immune system is comparatively stronger if the body has a collection of diverse foreign peptides to protect against more foreign pathogens.

“The more diverse the MHC of the parents, the stronger the immune system of the offspring,” said Yang. “The way one can recognize if someone has different genes is through scent.”

To test for attraction between two people, the researchers asked volunteers to smell the t-shirts of other subjects. The experiment from which the student models theirs only tested for attraction between females and males by making t-shirt wearers only male. Hussain removed gender from their experiment, however, by keeping the identities of the t-shirt wearers anonymous.

53 students volunteered as t-shirt smellers and ten students volunteered as t-shirt wearers. Wearers smeared their scent on the t-shirt by wearing it the previous night and giving it back to the researchers the next morning. Smellers were given a questionnaire asking basic questions, and asked to rate the pleasantness of the smell of the ten t-shirts, from 1-5. Hussein took saliva samples from each person’s mouth from which Yang extracted the DNA, incorporating knowledge from her seminar. “Since I don’t have much background in lab work, I was learning a lot about molecular techniques,” Yang said.

The next steps involve sequencing the amplified region of the gene at a lab at University of Pennsylvania. With those results, the students will use a computer program to analyze the relationships of the bases of the DNA for their different hypothesis and conclude their research by the end of the semester.


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