engin akyurt/UnsplashThere’s arguably no single hormone that has grabbed hold of the collective consciousness as much as cortisol.Social media is overflowing with information about the evils of cortisol: it messes with your face, your belly, your sleep, your skin, and, perhaps most concerning, your fertility.There’s no shortage of gimmicks available to help fight this hormone: cortisol masks, cortisol diets, cortisol watches, cortisol-suppressing supplements. But the widely held view that cortisol always causes problems with reproduction hinges on a key assumption that hasn’t been tested. Our new review, published in Journal of Experimental Biology, provides evidence that this assumption is flawed. In fact, cortisol may actually promote healthy reproduction. The hormone we love to hateCortisol has been cast as a modern health villain, seeking to slowly destroy you. But it’s actually a critical hormone – one of a few hormones your body can’t live without.This is because cortisol affects almost every cell in the body and helps to regulate many essential aspects of physiology, including circadian rhythm, metabolism and immune function.Importantly, how cortisol affects you is governed by its pattern of release: the ups and downs. Social media will have you believe a cortisol “spike” is always bad news.But precisely timed increases in cortisol are not necessarily bad – they may be exactly the signal your body needs to start one of its physiological processes, such as regulating your sleep.The long shadow of early cortisol researchDiscovered less than 100 years ago, cortisol is a relative newcomer to biology. At the time, researchers had no reliable way to measure natural cortisol levels in blood. But what they could do was manufacture synthetic cortisol, inject it, and observe the effects. Many studies on cortisol’s effect on reproduction used injections of synthetic cortisol. However, our bodies don’t always metabolise synthetic hormones in the same way as naturally occurring hormones. Furthermore, these injections were done without knowledge of normal hormone patterns. Cortisol was linked with stress early on, which meant that its role in other aspects of physiology was overlooked. Without actual data about normal cortisol patterns across reproductive stages, researchers assumed cortisol levels remained low unless they were spiked by some kind of stressor.Doing manipulations before you understand a system makes it difficult to interpret the results. Concluding cortisol is bad for reproduction based on injecting high doses is like taking a blowtorch to your dinner and then concluding that heat is bad for food. Read more: Cortisol ‘spikes’ are normal, so when is cortisol a real problem? What we discoveredWhat if cortisol is involved in reproduction? What if you need precisely timed increases and decreases to coordinate reproductive events?To find out, we conducted a large systematic review across the animal kingdom, including studies in mammals, birds, reptiles, amphibians and fish. We searched the literature for any information about normal cortisol patterns across the ovarian cycle. Most studies failed to consider natural cortisol patterns. Of the roughly 3,000 studies we screened, only 68 reported information about natural levels of cortisol during different reproductive phases. However, of these studies, 73% reported a significant change in cortisol during healthy ovarian cycles. Across species, cortisol levels often increased around the time of ovulation.Our results show that increases in circulating cortisol seem to play an important role in driving key reproductive events, such as ovulation.Rethinking cortisol’s role in reproductionThis information changes the way we interpret previous studies on cortisol’s role in reproduction and fertility.Large, imprecisely timed doses of synthetic cortisol may inhibit reproduction not simply because cortisol is “bad”, but because these manipulations scramble the body’s normal cortisol signals.If natural cortisol is trying to deliver a set of finely tuned physiological instructions, the synthetic cortisol injections are like someone shouting gibberish over the top. The message gets lost.Recognising that increases in cortisol may facilitate aspects of ovarian function supports a paradigm shift in how we think about the relationship between cortisol and reproduction. This challenges decades of thinking and opens the door to new explanations of, and treatments for, infertility.It’s also a cautionary lesson on not skipping steps in the scientific process. We should always carefully observe how a system works before manipulating it. Studies using synthetic cortisol contributed to a misleading narrative about this hormone – and we’re finally correcting the record.Kerry Fanson receives funding from La Trobe Tracey Banivanua Mar Fellowship. Phoebe Edwards received funding from Mitacs Canada and the Toronto Zoo.