Residents of Massachusetts may refer to having shared custody with their child's other parent rather loosely. The term shared custody actually has a technical meaning in Massachusetts law and can even refer to more than one concept.
A couple that is truly sharing custody in all respects will have an arrangement in which both parents spend a lot of time with their children on an ongoing basis and do so frequently and regularly. One should notice, however, that the law does not command that parents share time with each other on a perfect 50-50 basis, even if they have shared physical custody.
However, even when one parent has what is called sole physical custody, that is, the primary responsibility for caring for the children on a day-to-day basis, a parent can still share what is called legal custody with the other parent. As an aside, even when a parent has sole physical custody, the other will normally get some reasonable visitation, but not to the same degree as a parent with shared custody.
Parents who share legal custody have the same degree of responsibility, and say-so, when it comes to important matters in the child's upbringing, such as medical care, education and religious persuasion. On the other hand, a parent who has sole legal custody makes these decisions for their child, even if the other parent is expected to be reasonably informed about them.
In many cases, a parent is going to want to fight to have shared child custody, primarily so that both parents have the opportunity to keep their relationships with their children. On the other hand, in other cases, it may be important for a parent to fight to have as much exclusive authority over their child as possible.