Almost everyone crosses their legs, whether it's conscious or unconscious, for custom, for comfort, for effect, to stop your legs splaying, to take pressure off a foot, or for no reason at all. But is it bad for your health?
Your doctor will often tell you to uncross your legs because blood pressure rises slightly when you cross your legs at the knee. Some people are even able to use this increase in blood pressure to their advantage by crossing their legs to prevent an impending faint. But while high blood pressure is bad for your health, there is no evidence that leg crossing contributes to the condition.
Varicose veins are another reason we're told not to cross our legs. These unsightly, swollen blood vessels occur as a result of damage to the small valves that normally keep the blood moving in one direction: straight back to the heart. In those with varicose veins, some blood refluxes out into the small veins on the surface which subsequently balloon under the pressure.
Varicose veins are more common in women, especially those who have had (many) children and with advancing age. Crossing your legs may draw attention to varicose veins but it's not their cause.
Leg crossing has also been suggested (mostly by chiropractors) to lead to bad posture and its downstream effects on the back, hip and pelvis. Certainly, those with back and hip problems may experience discomfort when crossing their legs. Try it yourself and you may feel the muscles of your back tightening a little. If those same areas were otherwise inflamed it's easy to understand why leg crossing could seem problematic.
But what is cause and effect is unclear, and it may be that sitting in bad chairs for long periods is more to blame than what we are doing with our legs to cope. There is also some evidence that leg crossing could reduce strain on abdominal muscles and improve joint stability, which could actually be beneficial in some cases.
There's some evidence to suggest leg crossing could reduce strain on abdominal muscles. Flickr/joejukes.
Prolonged compression of the (peroneal) nerve that runs along the outer part of your knee can sometimes make your foot "fall asleep" after crossing your legs. This is not dangerous or a sign of impending paralysis, and after a few seconds things will usually return to normal.
In a very small number, prolonged and/or habitual leg crossing may damage the nerve, and probably has more to do with a special susceptibility rather than the behaviour itself.
In men, crossing your legs while wearing trousers raises the temperature of the groin. This has led to the suggestion that would-be fathers should not cross their legs.
But you'd probably have to leave them crossed for many hours every day to have any effect on your sperm count. And besides, most men tend to rotate their hip out when crossing their legs for long periods to make a "figure four" and thus alleviate any unwanted tension.
Leg crossing has long been linked to morality and etiquette. In some countries and cultures leg crossing is looked on as casual, disrespectful, and altogether lower class. For the same reasons, many orthodox religions frown upon leg crossing in church. And what's good for the soul should be good for the feet as well.
But piety and respect aside, you're unlikely to do any long-lasting harm simply by crossing your legs while sitting. The problem is really caused by sitting in the first place. Don't make yourself comfortable - get up and get moving.
Merlin Thomas is an adjunct professor of Preventive Medicine at Baker IDI Heart & Diabetes Institute.