Description: The birthday rule is used to determine whether the father's or the mother's insurer is liable for a child's medical bills, where the liability rests with the insurer of the parent whose birthday falls in the first half of the year. It is assumed that the odds are even that the liable insurer will be the mother's, vs. the father's. This paper demonstrates how the key determinant of whether this assumption is valid is the distribution of the difference in spouses' ages.