When
a baby cries at night, exhausted parents scramble to figure out why.
He’s hungry. Wet. Cold. Lonely. But now, a Harvard scientist offers more
sinister explanation: The baby who demands to be breastfed in the
middle of the night is preventing his mom from getting pregnant again.This devious intention makes perfect sense, says evolutionary biologist David Haig, who describes his idea in
Evolution, Medicine and Public Health.
Another baby means having to share mom and dad, so babies are
programmed to do all they can to thwart the meeting of sperm and egg,
the theory goes.Since babies can’t force birth control pills on
their mothers, they work with what they’ve got: Nighttime nursing
liaisons keep women from other sorts of liaisons that might lead to
another child. And beyond libido-killing interruptions and extreme
fatigue, frequent night nursing also delays fertility in nursing women.
Infant suckling can lead to hormone changes that put the kibosh on
ovulation (though not reliably enough to be a fail-safe birth control
method, as many gynecologists caution). Of course, babies don’t
have the wherewithal to be interrupting their mothers’ fertility
intentionally. It’s just that in our past, babies who cried to be nursed
at night had a survival edge, Haig proposes.
The timing of night
crying seems particularly damning, Haig says. Breastfed babies seem to
ramp up their nighttime demands around 6 months of age and then slowly
improve — precisely the time when a baby would want to double down on
its birth control efforts.Genetic disorders that are inherited
from mothers or fathers provide even more evidence. Babies who get
certain genes from their mothers sleep longer in the night, which is in
the best interests of a woman who wants to get pregnant again. But
babies who get the same genes from their fathers wake up more often,
delaying ovulation in their mothers, Haig writes. That makes
evolutionary sense: Because fathers have no guarantee that the next baby
will also be his, they (men and their genes) are presumably not
interested in ovulation starting again.Haig’s work builds on a
similar proposal
published in 1987, and if it’s right, it means that breast-fed babies
who cry at night might be showing the ultimate sibling rivalry.
Unsurprisingly, Haig has received a lot of interest on his theory, from
the general public (he just
appeared on Fox News) and from other scientists, who wrote
responses to his work in the same journal.In his comment, anthropologist James McKenna of the University of Notre Dame
points
out that infants may have evolved to wake up at night for all sorts of
other good reasons. Babies can get too hot, or hungry, or they could
just want a cuddle from mom. And babies aren’t always to blame for
rousing: In one study, McKenna and colleagues found that 40 percent of
babies’ night wakings were actually caused by mothers rustling around
nearby. These wakings could easily have benefits for the baby that trump
the birth control for mom, McKenna writes: Frequent wakings prevent the
baby from slipping into too deep a sleep, which can be dangerous. But
if Haig is right, the little screamers are doing all they can to
prevent another baby from coming along and ruining their good thing.
That self-interest is in direct conflict with the mother’s evolutionary
goal, which is to shove her genes into as many children as possible.
These divergent goals, Haig says, are an overlooked part of child-parent
relationships. “Mothers have evolved to maximize their numbers of
surviving children, which is different from maximizing the survival of
each individual child,” he says.
There’s no way to go back and
test whether night nursing actually helped babies survive in the early
chapters of our evolutionary history. Today’s babies are growing up in a
world that doesn’t look much like the one in which this trick could
have been useful. “I think that it’s an adaptation for a world very
different from the current world,” Haig says. Contraception, solid
nutrition and good health care have probably removed modern babies’
drive to prevent another sibling. Although we’ll never know
exactly why babies evolved to cry at night, Haig’s idea offers one
interesting explanation. Whether he is right or not, there is another
message lurking in this study, and it’s a message for modern parents:
Babies who don’t breastfeed during the night and babies who take bottles
don’t wake up as much during the night — and they don’t seem to be
worse off for it, Haig says. That result implies that nursing throughout
the night isn’t necessary. So moms shouldn’t beat themselves up if they
don’t always heed the nighttime calls to breastfeed, Haig says.
“There’s
a tendency to think of infants as incredibly fragile beings, and if you
do just one thing wrong, they’re ruined for life,” Haig says. “That to
me doesn’t make any evolutionary sense. They should be fairly robust and
handle all sorts of variation in sleeping arrangements and feeding
arrangements.” He wraps up our interview with a sentiment that I think all parents embrace: “Do what feels right for yourself.”
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