Establishing Credibility -- Updated with the corrected link

Oleg Volk, in addition to being possibly the best photographer of firearms on the planet, is a solid thinker.  He pinpoints the reason behind the mere presence of a firearm acting as a deterrent. 

If threatened by someone physically larger and stronger -- as is usually the case when males attack females, skill is not visible.  Muscle, weight, and physical presence are obvious.  A 98-pound woman may have the best Kung Fu there is, but she is going to have to use it if she is targeted by a 250-pound rapist.  On the other hand, if a woman or a fragile, elderly man had a gun in hand, an attacker may decide, as Falstaff, that discretion is the better part of valor.

This is why firearms are the best self-defense weapons in almost every situation.