IT'S FAIRLY traditional at this time of year to take out a new gym membership, full of good intentions for counteracting the excesses of the last few weeks. But ladies, as you're doing ab crunches to get rid of the flabby tummy, you might like to reflect on why it's such a big deal to have a small waist anyway.
Much research confirms that men find women with small waists relative to their hip size attractive – that is, those with an hour-glass figure. The preference doesn't seem related to body weight: even very large ladies may be deemed attractive if their waist is small in relation to their hips.
With such a universal preference, there must be a biological advantage. According to a study recently published in the journal Evolution and Human Behaviour, the importance of a small waist-to-hip ratio (WHR) lies in the development of part of the human anatomy out of all proportion to the rest of our bodies. I'm not talking about bottoms, I'm talking about our super-sized brains.
Fat stored in the buttocks and thighs contains omega-3 polyunsaturated fatty acids (known as DHA) which are essential for the brain development of foetuses, and women with low WHRs have higher levels of these fatty acids in their blood. Pregnant women need to have this type of fat available to nourish their growing baby. As girls move into puberty, fat gets deposited in these areas preferentially, accounting for the differences in body shape between men and women. This gluteofemoral fat isn't used by the body until late pregnancy, which would explain why it's so hard to streamline those wobbly thighs even when on a punitive diet.
Feeding our babies' brains explains the importance of a fulsome derrire and thighs, but what about that small waist? It turns out that abdominal fat, which tends to be saturated fat, is bad news – it is linked with diabetes, obesity and increased risk of heart disease. Abdominal fat also appears to inhibit the enzymes used to synthesise DHA, so the more there is of it, the less DHA is available to feed that growing baby's brain.
These considerations led William Lassek of the University of Pittsburgh, Pennsylvania, and Steve Gaulin of the University of California at Santa Barbara to deduce that a woman's body shape could be related to the intelligence of her children.
When the researchers examined data from a large-scale survey carried out by the US National Centre for Health Statistics, including 16,325 women of all ages and a variety of racial types, they found that children of women with lower WHRs had, on average, higher scores in a range of cognitive tests, even when factors such as education and family income were taken into account.
Not only that, but women with lower WHRs tended themselves to have higher test scores in intelligence tests.
Cognitive ability is very complex and different factors contribute to a person's general intelligence. According to psychologist Timothy Bates of Edinburgh University, intelligence is largely inherited.
"The biggest factor in intelligence differences by far is genes," he says. "Studies in many thousands of twins indicate that 70-80 per cent of differences between adults are genetic."
According to Dr Lassek, the effect of WHR and the resulting availability of DHA comes into this heritable component to some extent. "The role of DHA is probably partly genetic," says Lassek, "with some women having a more genetically endowed ability to make, conserve, and store DHA."
The availability of DHA decreases with each successive pregnancy, say Lassek and Gaulin, and they suggest this can explain the effect that birth order has on cognitive scores, with later-born children doing less well. Twins also score lower in tests, on average, than singletons.
Alarmingly, the size of women's brains also decrease during pregnancy – which gives us a reasonable excuse, I think, for losing the ability to remember what we're talking about when in the later stages of gestation (if you're anything like me).
"Lower body fat is depleted by pregnancy and nursing," says Lassek, "and even when abundant food is available afterwards, it is less likely to be replaced (than abdominal fat]."
This would explain many mums' issues with expanding waistlines and bottoms that are less pert than they used to be.
So all this has led Lassek and Gaulin to suggest that the reason men have evolved a preference for women with hour-glass figures is because such a body shape indicates the availability of critical resources for the brain development of any children they might sire with such a woman. And a low WHR also indicates that the woman in question is less likely to have already had children with another man.
Such a body shape may have a further advantage in that it could mean that it's easier to get pregnant. Graznya Jasienska of Jagiellonian University in Krakow and her colleagues carried out a study of Polish women looking at the link between body shape and circulating levels of the reproductive hormones oestrogen and progesterone. They found that women with low WHR had higher levels of both hormones, and this signals higher potential fertility.
So, a full bottom and hips along with a flat tummy and narrow waist is the way to go – if you want to attract your man of choice and have smart kids, that is. No wonder Trinny and Susannah have done so well with their magic knickers.
Anyway, next time someone says your bum looks big in those jeans – take it as a compliment. And keep going with the ab crunches.