Currently, by the looks of it here in Japan, it's more like 95% are born into the organization. This is based on the actual ones being baptized at the assemblies and conventions. They are all teens and young adults who were raised as Jehovah's Witnesses from childhood on. I suppose the other 5% are those who eventually had succumbed to the pleads of their JW family members. I found that many JW Japanese households don't have their "fathers" in the organization. Japanese men are "work-a-holics" and are devoted to the companies they work for. These "fathers" of JW Japanese households are embarrassed if they were to reveal to their peers that they are passively associating with Jehovah's Witnesses, let alone to reveal that their own families are JW's (pioneers for that matter). Here in Japan, JW's are viewed as a cult, as like in many other places. And since there are many cult-like factions popping up here in Japan, the JW's are hard pressed for finding "other sheep" who will even listen to their message.
Alligator Wisdom (aka Brother "NOT" Exerting Vigorously")