The big factor that tends to determine how parental responsibilities are managed is the level of conflict between parents. Parents who tend to get along relatively well after separation and tend to agree about the bigger decisions in the children's lives will almost certainly share all parental responsibilities, likely with neither parent having the final say about decisions. When parents don't get along at all, and can't talk about anything without getting into a yelling match, that's when you're likely to see parenting parental responsibilities being divided up with one parent having sole responsibility for some or all decisions.