It’s been blamed for polluting our rivers, destroying our marriages and – ironically – killing our sex drives. It’s been called a conspiracy by the patriarchy. It’s even be accused of making women fancy ugly men. But for many women, one of the most off-putting charges against the contraceptive pill is that it makes us fat.

In fact, weight gain is the most commonly reported side effect of the combined pill – the most popular type, which contains both lab-made oestrogen and progesterone. This is why pharmaceutical companies list it on the packet.

But after several decades of research, there still isn’t any conclusive evidence that the effect is real. The largest review so far examined 49 studies of the combined pill and found “no large effect is evident”, but also that there wasn’t enough well-conducted research to be sure. The researchers found this is true no matter what type of progesterone the combined pill contained (for more on the different kinds of pill, check out this article). Other studies that looked at progesterone-only pills similarly have found little evidence of an effect.

You might also like these other stories in the Health Gap:

• The strange truth about the pill

• How the menstrual cycle changes women’s brains

• Why isn't this type of birth control used more?

Maria Gallo, an endocrinologist at Ohio State University who co-authored the review, believes our belief in the pill-weight connection is down to a natural human bias. People are experts at finding patterns all around us, even where there are none. The phenomenon, known as apophenia, is the reason we may find it easy to compare peppers to politicians, see the faces of Harry Potter villains staring down at us from the clouds, or read too much into past lottery numbers. We’re particularly susceptible to apophenia if we’ve been primed to expect a certain outcome – such as gaining weight after starting a new medication.

“It’s the same reason why there’s this idea that vaccines can cause health problems,” says Maria Gallo. “If you give them to a population, you’re going to have some people who have health problems, whether they’re linked to the vaccine or not.”