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"Two years apart is ideal." "Three years apart and they won't fight." "Irish twins are exhausting, but you get it all over with." The opinions on sibling spacing are plentiful — from your own parents, from the waiting room at the pediatrician, from every parenting book that dares to take a position.
With this much folklore on the table, it's easy to assume that a correct answer exists. What does the evidence actually say? Researchers in economics, developmental psychology, and perinatal medicine have each approached sibling spacing from a different angle, and the picture they produce together is considerably more complicated than "two years is best."