NEW YORK -- Here's news to wrinkle your nose: In a study using armpit secretions, scientists have found what they call the first proof that people can influence each other through airborne chemical signals they don't even notice.
When researchers wiped the secretions from one group of women under the noses of other women, the second group showed changes in their menstrual cycles. The cycles got either longer or shorter, depending on where the donors were in their own menstrual cycles.
The affected women said they didn't smell anything but alcohol put on the pads. The alcohol alone had no effect on the women's menstrual cycles.
Nobody has identified the underarm substances that produced the effect, but once that happens, they might lead to new contraceptives and infertility treatments, said Martha K. McClintock of the University of Chicago, who reported the findings with a colleague in Thursday's issue of the journal Nature.
The work adds to indications that people, like animals from insects to elephants, influence each other by giving off chemical signals called pheromones (pronounced FAIR-o-mones). In animals, pheromones do such things as block pregnancies and influence mating preferences, timing of puberty and dominance.
The range of effects in people is still an open question. It's known that newborns and their mothers can recognize each other's body odor, for example, but scientists disagree on whether that counts as a pheromone signal.
In any case, McClintock's paper "will stand up as a classic in the field," said George Preti, a researcher at the Monell Chemical Senses Center in Philadelphia who has done similar research.
While some experts cautioned that the results should be considered tentative until they are confirmed, others called the findings convincing.
"The work is pivotal," said Charles Wysocki of Monell. "It basically says, `Look, people, we are influenced by pheromones that emanate from other people."'
Aron Weller, a psychologist of Bar-Ilan University in Ramat-Gan, Israel, who wrote a Nature commentary on the paper, said in an interview that the study is "the most convincing and clear-cut finding of human pheromones."
But he said it's not clear yet whether these pheromones have any effect in everyday life. Past studies -- including some by McClintock and Weller -- have shown that the cycles of women who live together can synchronize, especially between close friends. Pheromones probably cause that, but the link isn't yet proven, Weller said.
McClintock's study found that the secretions affected the timing of ovulation, which is the production of mature eggs. Once scientists identify the compounds responsible and the details of how they work, they might be able to create new compounds that suppress or encourage ovulation, McClintock said. Such compounds might be useful as contraceptives or treatments for some cases of infertility, she said.
Her experiment involved nine donor women and 20 recipient women in their 20s and 30s. The donors bathed without perfumed products every day and then wore cotton pads in their armpits for at least eight hours.
Later, pieces of these pads were wiped under the noses of the recipient women every day for four consecutive menstrual cycles. The women weren't told what the researchers were studying.
For two cycles, each recipient got secretions produced late in the follicular phase of the donors' menstrual cycle, which precedes ovulation. For the other two cycles, each recipient was given secretions produced at the time of ovulation.
The recipients' cycles were shorter than normal when they were exposed to follicular-phase secretions, and longer than normal when they got ovulatory secretions. Each effect amounted to about 1.5 days' difference from the norm on average, but ranged up to 14 days, and appeared in about two-thirds of the women.