In recent decades, there has been a bit of a shift away from always giving custody to mothers, as courts have decided that mothers and fathers should both be involved in the lives of their children. This has led to many joint custody cases where children move back and forth from one home to the next.
When looking at this shift, it's interesting to put it into a historical perspective, looking at the way that custody roles have been viewed over time and the impact this has had on culture.
For example, back in the colonial era, men were almost always given custody rights. Fathers were thought of as the heads of the households, in charge of almost everything. Even the manuals that helped with raising children were not aimed at women, but almost always at men.
In the 18th century, though, this started to change. Courts begin to note the role that women had in raising kids, and they tried to find more balance. Men were still thought to have a better economic standing, but this wasn't the only thing that was considered.
This trend continued into the 19th century, when two important things happened. First, public ideas of children as property began ending, as children were seen as individuals, and women began to emerge as being in charge of the home. As such, women began to be seen as the natural choice for raising children, and they got custody more and more often.
As you can see, these trends have been shifting for centuries, bringing about the modern age when mothers and fathers are thought of as equal in custody cases. If you're involved in such as case, you must know your rights to be involved in your children's lives in California.
Source: Child Custody Project, "Child Custody and Popular Media," J. Herbie DiFonzo, accessed Feb. 11, 2016