You might credit the legendary Scottish male virility of past time to single malt whiskey, or the sometimes brutal weather, or the fact that haggis is the national dish, but a Dutch researcher is proposing another answer:

"Men wearing a kilt experience a strong sense of freedom and masculinity," says a researcher. Here Tim Propst, of Lincoln County, N.C., throws a hammer during the Grandfather Mountain Highland Games in Linville, N.C. in July 2012.
It was the kilts.

Kilts, worn as they were meant to be worn, without underwear, lets our laddies swing freely in the breeze, creating, according to researcher Erwin Kompanje, the “ideal physiological scrotal environment.” Exposed to the bracing Highland coolness, testicles will make robust sperm.

The modern man’s “scrotal environment” is pretty confined these days, what with underwear and pants that hold our testicles close to the body and its 98.6-degree heat, Kompanje, a senior researcher in the department of intensive care at Erasmus University in Rotterdam, told NBCNews.com. But as he pointed out in a paper published online in the Scottish Medical Journal, “adequate spermatogenesis requires a temperature about 3 degrees lower than normal body temperature.” (That would translate to about 93 degrees Fahrenheit, compared to the normal body temperature of 98.6 Farenheit.)
 
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