There's been increasing evidence from both large-scale and smaller studies that the pill has a multitude of effects on women's health, their bodies, brains, and wellbeing. Now, a new study suggests oral contraceptives could also have an impact on women's social judgement.

If you are one of the millions of women currently taking the pill, there's no need to start freaking out. As cognitive psychologist Alexander Lischke explained to ScienceAlert, the effects his team observed are just 'subtle impairments', so mild you'd probably be unaware of them.

The issue appears to involve the way we identify another person's feelings by looking at their face. The participants of this study were given an emotion recognition task, and the team discovered a subtle effect in the group who were on the pill - the participants struggled with some of the trickier emotions presented.

It's an effect that's been mirrored in past studies, although we do want to note that the sample size was rather small, only 94 participants in Germany. And, when it comes to the emotional impact of the pill, research on this topic is still in its infancy.

That might be surprising, but it's true. Around the world roughly 100 million women use contraceptive pills to prevent pregnancy or control their menstruation.

Yet even though these drugs are some of the most extensively studied in the history of medicine, today we know surprisingly little about their effect on a woman's thoughts, emotions or behaviours.

That's because most of the research so far has focused on the physical health effects. These are no doubt important, but it's also true that the most common reason women stop or change the pill is because of mental health side effects.

Lischke, who works at the University of Greifswald, says there's now an increasing body of research investigating the mental effects of these drugs. Although with mixed results and less than ideal methods, the conclusions we can draw from this pile remain flimsy.

So far, most of the psychological research has focused on mood and cognition. Lischke's study is among the first to look at the consequences for emotion recognition and control.

One of the only other studies suggested that the pill might impair a woman's ability to process her own emotions and to have empathy for others.

"If oral contraceptives caused dramatic impairments in women's emotion recognition, we would have probably noticed this in our everyday interactions with our partners," explains Lischke.

"We assumed that these impairments would be very subtle, indicating that we had to test women's emotion recognition with a task that was sensitive enough to detect such impairments."

To that end, the researchers tested 41 women who currently use oral contraception and 53 women not on oral contraceptives. Before they began, the participants answered questions about their menstrual cycle, contraceptive use, age, education, distress, and empathy.

They were then given a test called 'Reading the Mind in the Eyes' - testing them on their ability to read subtle social cues in black and white pictures of just people's eyes. The participants were shown a variety of intense expressions, from minimal difficulty to moderate to maximal.

An example of an item from Reading the Mind in the Eyes test. (Kawata et al., 2014)

The results were subtle but significant. While the groups were equally good at recognising easy emotional expressions, women taking the pill were nearly 10 percent worse at figuring out the most complex ones.

This was true regardless of the participant's menstrual cycle phase or the type of oral contraceptive they were taking.

Lischke says these results compare nicely to previous studies, which have found that oestrogen and progesterone levels could affect a woman's emotion recognition.

"Since oral contraceptives work by suppressing oestrogen and progesterone levels, it makes sense that oral contraceptives also affect women's emotion recognition," he explains.

"However, the exact mechanism underlying oral contraceptive induced changes in women's emotion recognition remains to be elucidated."

Reading facial expressions is a crucial part of human relationships and one of the only signals we have about the feelings and intentions of others.

The question is: are these observed impairments powerful enough to cause interpersonal difficulties for women? We still need a lot more research to tell - specifically studies with larger sample sizes, harder tasks, and extended time spans.

"We do not know yet whether [oral contraceptive-related] impairments in emotion recognition have serious consequences on women's social life," Lischke told ScienceAlert.

"We know that women who are suffering from mental disorders like, for example, borderline personality disorder or major depressions, show marked impairments in emotion recognition that account for their interpersonal difficulties. However, the impairments these women show are far more pronounced than the impairments that we observed in women that were using [oral contraceptives]."

There's still so much we don't know, and that alone seems like a pretty good reason for more research.

This study has been published in Frontiers in Neuroscience.