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WE LIKE to think we are in charge of our own behaviour 鈥� that our thoughts are under our conscious control and that our actions are mostly reasonable. But our behaviour is also in the sway of an ancient system of mind control: hormones. These protein messengers are best known for their fundamental duty as regulators 鈥� think of insulin and blood sugar, for example 鈥� but they also bathe the brain in chemical information that tells us about the world around us and the people in it.

Can a surge in a particular hormone make us feel and act like a totally different person? And if so, are we right to blame our out-of-control moments on some kind of biochemical signalling? Here, we look at some of the big notions about how hormones mess with your head and sift fact from fiction.

Oxytocin equals love

Of all the hormones, oxytocin undoubtedly has the best PR. Widely known as the love hormone and the cuddle chemical, it has a reputation for spreading goodwill among humankind. It has even been touted as a potential treatment for autism, anxiety, depression and chronic pain.

Released during childbirth, breastfeeding and orgasm, oxytocin induces maternal and mate-bonding behaviour in many animals, including ourselves. In 2005, the possibility of hacking this system first reared its head when researchers found that people given an oxytocin nasal spray were more likely to trust others around them. Subsequent studies have found that sniffing oxytocin increases generosity, cooperation and empathy. Now the sprays are sold online, promising to .

But not everyone believes the hype. Reviewing the evidence in a paper published last year, Mike Ludwig at the University of Edinburgh, UK, pointed out that , and that even the original researchers are backing away from its conclusions.

Nor has it been proven that oxytocin can cross the blood-brain barrier. Studies of cerebrospinal fluid taken from people who had sniffed the hormone just beforehand suggest that it might, but it is too early to say for sure, Ludwig says.

Even if oxytocin does indeed enter the brain, its effects appear to depend on context. Studies in mice suggest it . Transplant such an effect to humans鈥� complex social lives and it could be a double-edged sword, promoting group bonding but perhaps also . Other studies suggest that . All things considered, it might be best to get our warm and fuzzy feelings from real cuddles for the time being. Jop de Vrieze

Periods make you see red

The comedian Roseanne Barr once said, 鈥淲omen complain about PMS, but I think of it as the only time of the month when I can be myself.鈥�

When premenstrual syndrome causes a woman to morph into an angry, tearful or irritable version of herself, it鈥檚 pretty clear that hormones are to blame. Which ones and what they are doing to the brain is less clear.

Part of the problem is that the menstrual cycle involves four hormones, each peaking at different times (see graph). Subtle differences in their fluctuations and in symptoms from one woman to another make it far from simple to pin down the effects of any particular hormone.

The main players in the cycle are oestrogen (specifically a form of it called oestradiol) and progesterone, both of which affect parts of the brain. High progesterone levels, for instance, have been linked to increased activity in the amygdala, a brain area involved in threat detection, which may explain why nerves can feel so raw in the week before a period.

Mood swings could also be the result of a sudden drop in oestrogen, high levels of which confer emotional resilience (see 鈥�The upside of the hormonal roller coaster鈥�). One intriguing possibility is that it鈥檚 not so much the drop-off that causes the bad mood as the unusual high that precedes it. 鈥淥estradiol seems to be responsible for a phase of good mood and well-being shortly before ovulation, when levels peak,鈥� says at the University of Salzburg, Austria. 鈥淭his lack of a peak has been discussed as one reason for negative mood changes in women on the pill鈥�, who don鈥檛 ovulate, Pletzer says. A slower decline in oestrogen before menopause also protects women from mood alterations, says , a neuroscientist at the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany.

Why some women seem more 鈥渉ormonal鈥� than others may have less to do with different levels of hormones than with . A study published early this year linked a person鈥檚 susceptibility to PMDD, a more severe form of PMS, with . Emma Young

Hormones make you hangry

Ever felt ready to fight for the last biscuit? 鈥淗anger鈥� 鈥� feeling angry due to hunger 鈥� is incredibly common, and ghrelin, the so-called 鈥渉unger hormone鈥�, is implicated. Released when the stomach is empty, it triggers a rise in levels of neuropeptide Y (NPY), a neurotransmitter involved in making us want to eat 鈥� and in regulating anger and aggression. People with intermittent explosive disorder, characterised by impulsive aggression, have .

What鈥檚 more, the more NPY in circulation, the greater the fall in the level of another neurotransmitter, . Low serotonin has been linked with reduced communication between the amygdala 鈥� the brain鈥檚 threat detector 鈥� and the prefrontal cortex, which regulates emotional responses. In such a situation, it might be .

But hormone and neurotransmitter levels alone don鈥檛 dictate whether you are likely to lose it when peckish. The amount of connectivity between prefrontal cortex and amygdala varies between individuals, suggesting that some people may be more predisposed to hanger.

Those who are can take comfort from the idea that it could well be an adaptive trait. 鈥淔rom an animal perspective, being angry 鈥� and possibly more aggressive 鈥� when you鈥檙e hungry can certainly increase the chances of survival,鈥� says Luca Passamonti, a neuroscientist at the University of Cambridge. EY

Cortisol is bad for you

Cortisol is widely thought of as the bad guy, a 鈥渟tress hormone鈥� linked to chronic health conditions, and therefore many think its level in the body should be lowered at all costs. Supplements are sold online that claim to help rid the body of the stuff.

This might not be such a good idea, however. Cortisol鈥檚 job is to trigger the release of glucose into the bloodstream. This happens first thing in the morning to provide a much-needed energy boost to get us out of bed. And in times of stress, it gives us the energy to respond to a mental or physical challenge.

In the right context, then, cortisol is definitely not bad for you. People with Addison鈥檚 disease, who produce too little of it or none at all, experience debilitating symptoms including fatigue, and require daily treatment to top up the hormone. On the other hand, too much cortisol in the long term affects the brain in a number of ways. It can impair the generation of new cells in the hippocampus, a region involved in memory, and is also implicated in depression.

鈥淚t鈥檚 not the absolute level of cortisol that matters, so much as the pattern of cortisol reactivity and recovery,鈥� says stress researcher at Columbia University in New York. Fortunately there is a simple way to achieve a beneficial pattern. Regular exercisers experience useful cortisol surges, says Stults-Kolehmainen, and levels drop quickly after the stress of exercise has passed. EY

Testosterone makes men angry and bald

Raging testosterone has been blamed for everything from wars to hooliganism to the banking crisis. Yet its reputation for putting men on the attack does not stand up to scrutiny.

A study published last year, , showed that although high testosterone levels are linked to status-seeking behaviour, the form that behaviour takes depends on social norms. While men given an injection of testosterone were more likely to punish someone who treated them unfairly in a game, they were also more likely to reciprocate if their opponent was generous.

As for baldness, contrary to popular belief, it isn鈥檛 a sign of high testosterone levels. Hair loss is down to an enzyme called 5-alpha-reductase, which converts testosterone to dihydrotestosterone, a compound that causes hair follicles to shrink and die. Just a small amount of testosterone is enough to make a destructive dose of dihydrotestosterone, and genetics determines both the amount of the enzyme a man produces and how sensitive his follicles are to its product.

The idea that falling testosterone causes a male menopause is also largely a myth. Testosterone falls by an average of 1 per cent per year after the age of 30, but only 2 per cent of men experience full-blown symptoms, including loss of libido, a drop in physical fitness and fatigue. In most cases, the cause is not the age-related drop in testosterone, but being overweight. Abdominal fat converts testosterone to oestrogen and it鈥檚 likely that this causes the symptoms, says , a urologist based in the Netherlands. JdV

Mums鈥� brains turn to mush

Parenthood changes everything, and at least some of that comes down to the way that a flood of hormones prepares the mother鈥檚 brain for the challenges ahead. Some of these changes happen in pregnancy, but far from turning the brain to mush, most are beneficial. One is an increase in the relative amounts of grey matter in brain regions involved in social cognition, . Women in the third trimester also have a reduced stress response, which protects the baby from high cortisol levels that can trigger early birth. It also means that soon-to-be mothers may be less likely to feel stressed than usual.

The changes don鈥檛 end when the baby arrives. In experiments, , taking just 50 seconds on average to find food hidden in their cage versus 270 seconds for rats that haven鈥檛 had pups. And human brain scans have shown that in the weeks and months following birth, areas of the mother鈥檚 brain involved in reward processing, reasoning, empathy and regulating emotion all bulk up. The researchers linked the changes to an , although how they bring these changes about isn鈥檛 yet clear.

, who researches maternal brain changes at Chapman University, California, says that it鈥檚 probably a combination of hormones making the brain more malleable, and sensory stimulation by the baby. It could be that, as in the case of oxytocin (see 鈥淥xytocin equals love鈥�), the hormones make mothers鈥� brains more sensitive to the world around them.

The effects can be long-lasting. Having children alters the mother鈥檚 hormone levels for decades, says at the University of British Columbia, Canada, so it鈥檚 not surprising that we see long-lasting effects on behaviour.

Fathers, too, show a rise in oxytocin and also in prolactin, which lowers testosterone. In one study, fathers had lower testosterone levels that other men of the same age. . Perhaps the hormone changes make men more attentive fathers. EY

Changing gender changes your brain

Hormones affect the brain and behaviour, so it stands to reason that gender transition, which begins with hormone therapy, would affect both. It鈥檚 true that there are anecdotal reports of increased aggression in people who have transitioned from female to male and thus have more testosterone than before, and of . But attributing such changes directly to hormones is tricky. 鈥淚t is difficult to disentangle potential direct effects of testosterone reduction from indirect effects of desired physical changes,鈥� says at King鈥檚 College London鈥檚 Institute of Psychiatry, who studies .

Nevertheless, there is some evidence towards what we see in people designated male at birth. SERT transports the neurotransmitter serotonin into nerve cells, and plays an important role in mood and anxiety disorders 鈥� the antidepressant fluoxetine (Prozac) for example, blocks its activity. Such disorders are more common in women, who have a naturally lower levels of SERT. This suggests that hormonal treatments might reduce mental health risks.

However, one of the researchers involved in the work, Georg Kranz at the Medical University of Vienna, Austria, says there are limits to hormone therapy鈥檚 effect on the brain 鈥� simply because so much of the brain鈥檚 wiring is laid down during development.

Brain scans have hinted at ways to identify what a transgender brain looks like, which suggests there is much still to learn about how genes, development and hormones work together to build a brain. EY

This article appeared in print under the headline 鈥淐hemical you鈥�