Ya know, I've spent a lot of time in MRA spaces, and as you know, this is a big one that comes up, and I was pretty fired up about it. I thought 50/50 made sense too.
Then I spoke to a (male) family attorney about it, and he said it really just depends on who provides the most care for the children before the divorce, and that's almost always the woman, even if both parents work full time. He says in cases where the man provides more care pre-divorce, custody reverts to him.
He went on to say that in a lot of cases, when they try to give the parent who provided less care pre-divorce 50% custody (yes, usually the man), within a few months, the men end up not taking it anyway, because they actually don't want the kid with them quite that much. Sometimes the women, wanting time to themselves, would insist the man take the kids for his allotted 50%, and he would refuse.
According to my attorney friend, in other cases, the parent who didn't give much care before the divorce (doesn't have to be a man) ends up being very neglectful during their "time" after the divorce.
I have to admit that I experienced this as a child - my dad had us in the summers and failed to feed us properly, didn't bathe us, and failed to provide supervision such that we were often either getting injured or (accidentally) destroying property.
The first summer we were with him, he forgot to go food shopping for the first week (and he was more often at his girlfriend's house than his own) and my brother and sister and I resorted to "eating" raw flour because that's all there was in the house. I was 5 and it was hard for me to control my breathing, so I often inhaled the flour and coughed it all out, which made my sister angry because I was wasting food. We ended up pooling our meager change and boarding a city bus trying to find a grocery store, only to end up in the terminal station downtown, and having the police call our dad (when he picked us up, it was the first time we'd seen him in several days).
My stepmom is fond of recounting the tale of the first time she met me - she and my dad returned from a date at 2am, and came into my dad's kitchen to find me, at 5 years old, standing on a chair in front of the stove, cooking a pork chop on high.
Remembering my own history, I had to admit that my attorney friend might have a point. I'm not saying that men are unfit parents, it totally depends on the man. But having been socialized to believe that taking care of children is not their responsibility, I think men are more likely to be unfit parents. It simply wasn't in my dad's programming that he might be responsible someday for the care and feeding of children.
I've asked him about it since, and he said, "It seemed like your mom was really controlling, so I wanted to give you kids some freedom."
I didn't really want freedom at age 5, though.
Anyway, wow, that got really long- maybe I should write a post about it.