A lot of girls, eager to calm the fears of their doting mothers and over-protective fathers, will exclaim, "I can take care of myself" after being warned about walking alone at night through a bad neighborhood, or some other equally-risky behavior. They're imagining themselves strongly and purposefully applying a kick in the balls to their would-be attacker. I hate to break it to you, ladies, but the tried-and-true kick in the balls isn't as effective a self-defense technique as a lot of women imagine.
A man's entire Raison d'etre, evolutionarilly speaking, is to keep the family jewels from harm. Evolutionary concerns notwithstanding, most mens' more immediate concerns are to avoid the incredible, disabling pain of a strike to the testicles - a pain which can only be described as a thousand times more intense than that of being poked in an eye. Consequently, men have spent their entire lives subconsciously protecting their crotch from any surprise attack. A woman attempting to kick a man in the 'nads without the advantage of surprise would stand little chance of connecting with anything other than a well-muscled thigh.
In the unlikely event of a kick that actually connects, the woman would more-than-likely be quite surprised when the man, unlike in the movies, does not immediately fall to the ground in disgrace and humiliation, but becomes completely enraged and, regardless of his original, perhaps somewhat innocuous intent, beats her to a pulp. They're called balls for a reason - they roll out of the way. A lucky kick might connect squarely with an unlucky testicle, but the odds of this happening seem to be quite low. A more likely result would be a painful, but not disabling, near-miss.
Ladies - don't pin your self-defense hopes on the knee to the groin or the kick to the balls. Carry a weapon in your hand whenever you're in a vulnerable situation and wear a good pair of running shoes. You do not have the ability to take care of yourself by standing toe-to-toe against a criminal who would do you harm. A man's testicles are not a magic button that you can push to disable him. They are well-protected and the odds of you hitting them are low.