[ubertakter]

The 50% claim probably comes from the fact that heat loss through your head has a greater affect on core body temperature than other exposed areas. In [1], they did some submersion tests with head in or out for both insulated and exposed bodies (test subjects). Head-in accounted for a fairly large increase in core body temperature loss (39-45% depending on the configuration). While this study was in water, it seems reasonable to expect the same thing in air, in particular since you tend to spend longer in the cold outside than you might in water.

Edited to add this effect is likely due to the amount of blood flow to the brain and the short path (relatively) between the brain and the heart.

[1] https://www.physiology.org/doi/full/10.1152/japplphysiol.012...