For the first time, researchers at the University of Chicago have compiled scientific evidence that secret chemical signals we emit called pheromones--which occupy a world apart from sight, hearing, smell, touch and taste--can affect human behavior as well as biology.
"They don't work directly, but can modulate how we feel and influence our moods," said biopsychologist Martha K. McClintock.
"A pheromone won't make you suddenly turn on the sidewalk and follow somebody's perfume trail, or stand there dumbstruck, unable to move--as they do, in effect, in animals and insects. But they definitely do have an effect on the emotional state of people."
McClintock's quest has been to prove the existence of a sixth human sense of which we are unaware but which causes us to respond to the environment in certain ways.
As a graduate student at Wellesley College in 1971, she published a paper in the prestigious journal Nature that documented menstrual synchrony in her dorm, the phenomenon in which the menstrual periods of women who spend a lot of time together sometimes begin to human pheromone synchronize.
Scientists immediately suspected it had to be due to undetectable chemical signals that were produced by one individual and could influence the biology of another.
Animals give off pheromones, which convey messages to others of their species, but scientists have found only sketchy evidence that humans do.
In 1998, immersed in a distinguished career, McClintock published in Nature experiments that demonstrated the existence of pheromones and showed how they could be used to control ovulation. Compounds swabbed from the underarms of young women at different human pheromonetimes of the month could alter other women's menstrual cycles, expanding or compressing them in predictable fashion.
"We had provided the first evidence that pheromones could affect physiology [the timing of ovulation]," she said Thursday. "Now, we wanted to see if they could affect behavior. And yes, we found that they are psychologically potent."
For the study published in the current issue of the journal Hormones and Behavior, McClintock and fellow researcher Suma Jacob tested men's and women's responses to undetectable amounts of two steroids produced by the body and often included in perfumes and colognes: androstadienone, which is produced by men, and estratetraene, produced by women.
Both were said to enhance the wearer's sexual attractiveness, with androstadienone supposedly encouraging women to become interested in sex and estratetraene having a similar effect on men.
"They had been reported to be sex-specific, in that the male steroid was to only affect women so it was part of a mating system. And the female steroid was believed only to affect men." McClintock said.
Both compounds are naturally produced in the body, though McClintock is not sure where. "They can be found in blood. And there's a group of scientists in Utah that claims they have been able to take it from the microscopic skin cells we all slough off each day. So that was a tantalizing idea to us."
U. of C. researchers tested 10 men and 10 women by applying a minuscule amount of each chemical under their noses and on a spot on their necks. The steroid was diluted in a chemical called propylene glycol, and the researchers also tested their subjects by applying that chemical alone to the same areas.
During the testing, the researchers conducted standard psychological tests to compare how the thoughts and moods of their subjects were influenced by exposure to the chemicals.
The experience was designed to be time-consuming and tedious, and the subjects were told they were studying various chemicals, including compounds used in perfumes.
McClintock described the testing technique: "Let's say that a woman comes into our lab and over time, things become more irritating and depressing. Her positive mood begins to drop because she's sitting in a small room working very hard.
"But we found that the tiniest dose of the steroid--either the men's or women's--would prevent her mood from slipping.
"On the other hand, we found that the steroids had the human pheromone opposite effect on men. They became less high, less euphoric, it was not a positive experience for them."
She said she was not surprised. "Men and women are very similar in many ways, but also very different. . . . In perfumes, these steroids are billed as sexual attractants, but we found they're not so, at least for men."
The development of colour vision may have led to Old World primates, and hence their human descendants, to lose their ability to detect pheromones, suggests a new genetic study.
Pheromones are highly specific scent molecules that many animals rely upon to find and assess a potential mate. But humans appear to make little, if any, use of pheromone signals, says Jianzhi George Zhang, an evolutionary geneticist at the University of Michigan, Ann Arbor.
Researchers have suggested before that the primates' pheromonal abilities may have fallen by the wayside because they developed colour vision, a better way of selecting mates. "But we establish the timing for when the pheromone signal transduction pathway was shut off," Zhang told New Scientist.
To the scientists, the results suggested that among group interactions, or being human pheromone close to other people in groups, there are unconscious chemical signals going on between people that affect their human pheromone psychological state and mood.
"We know, for example, that different kinds of odors change people's moods. The entire perfume and food industries make their living off that. But this is different.
"We're talking about things that can be sensed but not recognized or verbalized," McClintock said. "It's all subliminal.
"We're announcing a class of compounds that we're calling natural chemo-signals, if you will, that are produced by humans and can affect the psychological functioning of other humans. And not just in a simple sexual attractant system, but in a broader way to affect emotional states and mood."
McClintock hopes the study opens a new field to science, just as she identified the first two human pheromones in 1998.
"We have to find other compounds and determine what human pheromone roles they play in other situations. How do they affect people and their social interactions? Or problem-solving in groups? There are so many questions to ask."
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