The origins of this myth have been traced to the hat-wearing advice of an US army survival manual from 1970 which strongly recommended covering the head when it is cold, since “40 to 45 percent of body heat” is lost from the head.
Rachel Vreeman and Aaron Carroll, at the center for health policy at Indiana University in Indianapolis, dismiss the claim as nonsense in the British Medical Journal. If this were true, they say, humans would be just as cold if they went without a hat as if they went without trousers.
The myth is thought to have arisen through a flawed interpretation of a vaguely scientific experiment by the US military in the 1950s. In those studies, volunteers were dressed in Arctic survival suits and exposed to bitterly cold conditions. Because it was the only part of their bodies left uncovered, most of their heat was lost through their heads.
The face, head and chest are more sensitive to changes in temperature than the rest of the body, making it feel as if covering them up does more to prevent heat loss. In fact, covering one part of the body has as much effect as covering any other.
If the experiment had been performed with people wearing only swimming trunks, they would have lost no more than 10% of their body heat through their heads.
There may be a generalization of situations in which this myth is true, such as when the head is the only uncovered part of the body. For example, it has been shown that hats effectively prevent hypothermia in infants. However, conventionally, no more heat is lost through your head than through any other heat-sensitive part of your body, such as your face or chest, and it is certainly not 40-50% of your body heat.
[Reposted (with editing) from Misconception Junction]