Going shirtless helps men cool down in hot weather by allowing sweat to evaporate more easily.

The human body cools itself by sweating, which then evaporates from the skin and lowers body temperature. Wearing fewer clothes lets sweat evaporate faster. For men, who generally have more upper body muscle and sweat glands, going shirtless is an effective way to stay cool. This practical reason makes it common in warm climates and outdoor sports.