A flawed scientific experiment by the U.S. military in the 1950s is the widely recognized source of this popular myth. In the study, participants were subjected to extreme cold. The subjects were bundled up in survival suits, though their heads were left uncovered. The natural finding? Most heat was lost through their heads.
The truth is the amount of heat lost by any part of the body is generally in proportion to its surface area, though it may not feel that way. The face, upper chest and head are more sensitive to changes in temperature than other areas of the body.
It’s still important to wear hats and scarves as protection in cold temperatures, though.