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Mother Nature’s ‘optimal’ Birth Spacing


Due to economic and social factors, couples are waiting longer and longer to have their first child.  As a result, there has been a decrease in “birth spacing” (i.e., years between siblings).  What is the “optimal” amount of time between siblings?  One year?  Two years?  Five years?

Consider this theory from the world of Cultural Anthropology.  In an article entitled, “The Four-Year Itch,” author Helen E. Fisher found that divorce peaks at about 4 years.  Of course, there are some cross-cultural differences observed.  For example, there is relatively little divorce in societies where men and women are economically interdependent, such as in farming cultures, but there are high rates of divorce among cultures where men and women have economic freedom and/or opportunity (e.g., the Kung Bushman of the Kalahari Desert, where in order to divorce the women simply walk back to their family/tribe of origin; the United States in more recent decades when women have had greater educational and job opportunities).   Still, overall divorce rates tend to peak at about 4 years.

What may explain this “spike” in divorce is a biological process observed among mammals (like humans), who practice serial monogamy (i.e., one partner at a time, for a long duration, followed by another long-term partner).  Rewind 1,000 years, 2,000 years, or even a few thousand years. A man and woman meet and experience physical attraction. During the first few months of their relationship, they experience infatuation, frequent intercourse, and a baby is conceived. Nine months later the baby is born. Lactation occurs and supports the baby for 3+ years. The lactation serves to decrease the woman’s chances of getting pregnant again. At the end of lactation, the child is about 3 or 4 and has emerging independence (i.e., walking, communicating, ability to perceive/avoid danger, etc.).  The male nature is to propagate the species and spread his genes far and wide.  That being the case, the father takes off to find a new mate, leaving the mother to also find a new mate (i.e., serial monogamy).  About one year later, a new baby is born.  Thus, it is argued that “Mother Nature’s” pattern of birth spacing is about 4 or 5 years, which, incidentally, dove-tails neatly with the cross-cultural divorce data of “the four-year itch.”  It is also interesting to note that at 4- or 5 years of age most children have the security, mental capacity, emotional capacity, and “psychological room” to accept another child into the family

Surely, this theory may strike the modern romantic as far-fetched, as coldly academic, and may even offend sensibilities.  Still, it remains an interesting theory for purpose of discussion. Of course, the amount of time between siblings is a matter of personal preference, the “biological clock”, finances, and family planning, as well as unplanned events. Also, children are generally quite resilient and ultimately able to adapt to many variations of birth spacing.