Hormones: Learning the rules of attraction

A woman's smile may not be all it seems. When her zygomaticus major cheek muscle moves the upper lip upward and outward to produce that warm smile, it may be more than a friendly gesture – it could be a sign of hormones at work.

Researchers have found when a woman sees images of men, her smile muscle is more active during the follicular stage of the monthly cycle, and they suggest it may be a way of increasing the chances of intimacy. Progesterone, the hormone that prepares the womb for a possible pregnancy, is thought to be implicated.

It's the latest research to show the effects of hormones on mood and behaviour. Other hormones have been linked to depression, stress, anxiety, forgetfulness, social bonding, fatherhood, lying, generosity, romance and trust, as well as sexual relationships.

"No matter how sophisticated we think the expression of desire is, human behaviour is not so free from the action of hormones," says Dr James Pfaus of the Center for Studies in Behavioral Neurobiology at Concordia University. "What we are finding from studies is that hormones set a stage for human responses to environmental stimuli, just as they do in other animals."

Hormones are being used as the basis for treatments for conditions as diverse as stress, anaemia and memory difficulties, and are being investigated for others, such as symptoms of autism, obesity and depression.

Hormones – from the Greek hormo, to set in motion – are chemical messengers that travel around the body co-ordinating complex processes like growth and development, metabolism, fertility, and almost everything the body does to stay alive. They orchestrate the changes that occur at puberty, they affect the immune system, and they can alter behaviour. Secreted by a network of endocrine glands and distributed through the bloodstream, they enable communication with distant organs to co-ordinate the body's actions and reactions in events as diverse as disease, pregnancy and stress.

Research is increasingly uncovering the roles played by hormones in behaviour and mood. In many cases, it's being found that hormones have more than one role. Oxytocin, for example, which stimulates the contraction of the womb and milk ducts in the breast, has been shown to play a role in social bonding. "It produces strong effects, and is the first treatment we have found for improving empathy and bonding," says Dr Adam Guastella, senior clinical research fellow at the Brain and Mind Research Institute, University of Sydney, who led the study.

A spray based on insulin, the hormone that regulates blood sugar levels, is being used to treat forgetfulness, while testosterone, which directs the development of male features, is being used to improve blood sugar levels in men with diabetes. Prolactin, the hormone that triggers breast-milk production, has been found at higher levels in fathers who bond well with their children; low levels of the hormone cortisol have been linked to antisocial behaviour in boys. Erythropoietin, or EPO, produced by the kidneys and stimulating the formation of red blood cells, is being investigated as a way to treat depression.

"While hormones play a huge part in physical development, it is being increasingly seen that they play key roles in mood and behaviour," says Dr Nick Neave, reader in psychology at the University of Northumbria. "The two thyroid hormones, for example, are very closely linked to mood because people who have low or high levels can have major mood problems. There are probably hundreds of hormones, many of which we have yet to discover, which have an effect on everything that we do."

Potential new uses are also being identified through research on animals. Research at Cambridge University, based on meerkats, shows that males who were encouraged to baby-sit their offspring rather than forage for food had much higher levels of the hormone prolactin. The researchers say the finding lends weight to the idea that co-operative working in groups and families has a hormonal basis.

Insulin

What it is: Made by beta cells inside the pancreas, it helps the body use or store the glucose it gets from food. People with Type 1 diabetes, whose natural insulin is inadequate or absent, rely on insulin therapy.

What's new: A twice-a-day nasal spray of the hormone is being used to fight forgetfulness. It is designed to improve recall in people aged over 55 with memory difficulties, including dementia. Just how it works is not clear, but a theory is that it has an effect on nerve cells in the brain. "Acute insulin administration improves memory. Treatment with insulin has not been a viable option before," say researchers at the University of Washington.

Prolactin

What it is: Produced by the pituitary gland, it is secreted at higher levels during stressful events, pregnancy and breast-feeding. More than 200 effects on growth, reproduction and immunology have been reported.

What's new: It has long been known that the hormone plays a significant role in maternal care, but research is showing that it is connected with paternal care in humans, and in fish and birds. "It is important in paternal care and thus deserves the label 'hormone of paternity'," say University of Zurich researchers. Research at Emory University in Atlanta shows that new fathers with higher prolactin levels are more alert and positive in their responses to the cries of the baby.

Progesterone

What it is: Secreted by the corpus luteum and by the placenta, it's responsible for preparing the body for pregnancy and, in pregnancy, maintaining it until birth.

What's new: Research is increasingly showing that it has a big impact on mood. Watching a romantic film boosted progesterone levels by more than 10 per cent, bringing couples closer together, according to research at the University of Michigan. A movie such as The Godfather can alter testosterone and dampen attentive feelings. "When you're watching movies, your hormones are responding, not just your mind," said Oliver Schultheiss, who led the study. "This helps explain why certain people like to go to certain types of movies. Affiliation-motivated people like to see romantic flicks, but power-motivated people prefer movies with more action and violence. If you want to learn about someone's personality, look at their video collection or bookshelves."

Work at the Center of Mental Health, Marienheide, Germany, shows that the smile muscle of women was most active in the follicular phase of the monthly cycle, when the hormone levels are low. Women smiled more when exposed to images of men. "Up to now, no efforts have been made to investigate the relationship between ovulation-related shifts in women's sexual desire and changes in their facial expression of emotion. Smiling... is an important precondition of social relations of any kind. A female's smile at a male might increase the probability of a more intimate contact including sexual intercourse," the research found.

Cortisol

What it is: Secreted by the adrenal glands, it's involved in a wide range of functions, including glucose metabolism, regulation of blood pressure and immune system functioning, but is most associated with stress and preparing the body for the fight or flight response. Small increases of cortisol can result in a quick burst of energy for survival, increased brain activity and lower sensitivity to pain.

What's new: It could also have a use as a memory aid, according to University of Chicago research. Laboratory studies have shown that animals learn more quickly if they have a modest amount of cortisol than those with high or low levels. Those with high and low levels took an average of 14 attempts to find their way around a maze, compared to nine tries for the cortisol-treated group. A study at the University of Cambridge shows that stock markets affected levels of testosterone and cortisol in traders. When they were successful and made more money, testosterone levels were high, but when the markets were unpredictable, they had greater amounts of cortisol.

Oxytocin

What it is: Produced mainly in the hypothalamus, released into the blood through the pituitary gland, it stimulates contraction of the uterus and milk ducts in the breast. Research shows that breast-feeding women who have higher levels are calmer than bottle-feeders.

What's new: Also known as the hormone of love, it has been shown to increase social bonding. Sydney University research shows that oxytocin strengthens remembering of positive things. "It improves memory of positive interactions, and improves bonding in men, enhancing the development of positive relationships, and helps with social relationships," researchers say. The research has implications for treating some disorders where there are problems of social bonding, including autism and social anxiety. High levels have also been associated with increased trust and generosity. When researchers at Claremont Graduate University in America gave doses of oxytocin and a placebo to volunteers, those fed the hormone gave 80 per cent more money away to a stranger.

Oestrogen

What it is: Produced in the ovary, testes and placenta, this class of hormones has various functions in both sexes, including the development of the female sex characteristics. During the menstrual cycle, it acts to produce an environment suitable for the fertilisation, implantation and growth of the embryo.

What's new: Oestradiol, one of the oestrogens, also helps women to be more attractive to men, according to a Harvard study. Symmetrical-faced women, seen as more attractive by men, had a 21 per cent higher mid-cycle oestradiol level than asymmetrical women. At other times, the difference was as high as 28 per cent. "Our results suggest that, in women, symmetry is related to higher levels of oestradiol and, thus, higher potential fertility. As a consequence, men attracted to more symmetrical women may achieve higher reproductive success," the researchers say.

Oestrogen may help women to be safer drivers than men, according to Bradford University; researchers say the hormone may boost the part of the brain involved in attention span and mental flexibility.

Aldosterone

What it is: Produced by the adrenal glands, it acts on the kidneys to make sure that salt levels in the blood are at safe levels.

What's new: It is being investigated as a treatment for people who suffer from high blood pressure. Researchers from the University of Glasgow studied the way in which aldosterone affects blood pressure regulation and found that in older people, higher levels in the bloodstream are associated with high blood pressure, while in young adults, high levels indicate a greater risk of developing hypertension later in life.

Erythropoietin (epo)

What it is: A hormone naturally produced by the kidneys that stimulates the formation of red blood cells.

What's new: It is being used as a treatment for anaemia, and is being investigated as a therapy for depression. Tests on healthy volunteers show it does have an effect on brain chemicals involved in emotion.

Testosterone

What it is: A sex hormone that plays a key role in puberty, largely produced in the testes. In men, testosterone helps maintain bone density, fat distribution, muscle mass and strength, red blood cell production, sex drive and sperm production. It's involved in the development of male reproductive organs and features. Women have smaller amounts of testosterone produced in the ovaries, which plays a role in libido and maintaining muscle and bone strength.

What's new: Research shows that girls exposed to high levels of the male hormone in the womb have an increased interest in boys' toys like gun and cars. "They also show increased interest in boys' activities and in playing with boys," says Professor Melissa Hines of the University of Cambridge.

The hormone can raise male libido in a very short time. After just 4.9 minutes in the same room as a woman they'd not met before, and in some cases did not find particularly attractive, men's testosterone levels had shot up by an average of 8 per cent, according to research at the University of Groningen in Holland. The rising levels fuelled visible changes in male behaviour, including a squaring of shoulders, upright posture, and greater use of hands. Scientists at the University of Giessen in Germany found that in women, a lack of the hormone reduced spatial ability, including in map-reading.

Oxyntomodulin

What it is: A hormone released by the small intestine during eating, it may help people lose weight.

What's new: Imperial College research shows that those injected with it lost an average 2.3kg (5lb) in four weeks, five times placebo levels. Another hormone, ghrelin, is thought to tell the brain when it's time to eat.