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 improve your sex life, reduce anxiety and create feelings of trust.
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 no one has replicated the 2005 trust experiment, 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 alters brain circuitry so as to focus attention on socially relevant cues. Transplant such an effect to humans’ complex social lives and it could be a double-edged sword, promoting group bonding but perhaps also increasing hostility to outsiders. Other studies suggest that large doses of oxytocin may increase anxiety by making people oversensitive to what others say about them. All things considered, it might be best to get our warm and fuzzy feelings from real cuddles for the time being.
Ever felt ready to fight for the last biscuit? "Hanger" -- feeling angry due to hunger -- is incredibly common, and ghrelin, the so-called "hunger hormone", 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 -- and in regulating anger and aggression. People with intermittent explosive disorder, characterised by impulsive aggression, have above-average levels of NPY.
What's more, the more NPY in circulation, the greater the fall in the level of another neurotransmitter, serotonin. Low serotonin has been linked with reduced communication between the amygdala -- the brain's threat detector -- and the prefrontal cortex, which regulates emotional responses. In such a situation, it might be harder to rein in emotions when we are stressed.
But hormone and neurotransmitter levels alone don't 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. "From an animal perspective, being angry -- and possibly more aggressive -- when you're hungry can certainly increase the chances of survival," says Luca Passamonti, a neuroscientist at the University of Cambridge.
Is Cortisol The Bad Guy?
Cortisol is widely thought of as the bad guy, a "stress hormone" 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's 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's 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.
"It's not the absolute level of cortisol that matters, so much as the pattern of cortisol reactivity and recovery," says stress researcher Matthew Stults-Kolehmainen 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.
Testosterone Makes Men Seek
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, for example, 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't 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's likely that this causes the symptoms, says Herman Leliefeld, a urologist based in the Netherlands.