And Im sure the science of it has to take into account peoples skin and hair. Maybe some peoples hair reacts better to hot or cold. Etc. For it to be truly scientific you have to take everything into account. Genetics, skin sensitivity or lack of yada, yada , yada. Lather, type of brush, air temperature, how long it stays hot, (man my head hurts already). But its easier to just try both and see what works for you.