In boys, the scrotal sac is marvelously designed to maintain the testicles within a narrow temperature range, just below normal body temperature. When it is cold out, the sac constricts and draws the testicles close to the body for warmth; when hot, the sac is loose to keep them cooler.
When an undescended testicle is left untreated (especially beyond a year), its higher temperature causes an increasing risk down the road of infertility and testicular cancer.
Here’s the news — disposable, plastic-lined diapers keep testicles hotter than core body temperature (and as much as 1.8 degrees F higher than testicles in cloth diapers)! The October 2000 issue of Archives of Disease in Childhood speculates that perhaps the significant rise in male infertility over the last 25 years is due to the widespread use of diapers that keep kids too warm. This has certainly not been proven, but time will tell. For now, the reasoning makes a lot of sense to me.