Myth Buster: Do Women’s Periods Really Sync Up?

It’s a widely held belief, shared among friends and passed down through generations: when women live in close proximity, their menstrual cycles align. This phenomenon, known as “menstrual synchrony” or the “McClintock effect,” is often cited as prime evidence for the subtle power of human pheromones. But is it real?

The idea entered the public consciousness with a 1971 study by researcher Martha McClintock.^{[27], [42], [43]} She studied 135 female residents of a college dormitory and reported that over the academic year, the gap between the onset of their periods significantly decreased.^{[42], [43]} She hypothesized that this was due to unconscious chemical signals—pheromones—being exchanged between the women.^{[27], [42]}

The study was compelling and the concept was intuitive, quickly becoming a fixture of pop science. However, in the decades since, the scientific foundation for menstrual synchrony has crumbled.

The Scientific Rebuttal

Despite numerous attempts, subsequent studies have largely failed to replicate McClintock’s original findings.^{[43], [44], [44], [45]} Furthermore, critics have pointed out significant methodological and statistical flaws in the 1971 paper and others that supported it.^{[44], [45]}

The main arguments against menstrual synchrony are:

1. It’s a Statistical Artifact: Menstrual cycles are not perfectly regular. They vary in length from person to person and from month to month (a typical range is 21-35 days).^[43] Given this natural variability, the onset dates of any two women’s cycles will randomly drift closer together and then further apart over time. The original studies were criticized for failing to account for this chance convergence, leading to a false conclusion of synchrony.^{[44], [45]}
2. Lack of a Convincing Evolutionary Basis: There is no clear evolutionary advantage that would explain why human females would benefit from menstruating at the same time.^{[44], [45]}

The Verdict: A Statistical Illusion

Today, the overwhelming scientific consensus is that menstrual synchrony is a myth.^{[44], [44], [45]} The popular belief likely stems from a combination of mathematical chance and confirmation bias—we tend to notice the times our cycles overlap with a friend’s and forget the many times they don’t.^[45]

While the idea of pheromones powerfully orchestrating our biology is fascinating, the case of menstrual synchrony serves as a crucial reminder: scientific claims require rigorous, repeatable evidence. In this instance, the evidence simply isn’t there.