Self preservation is a strong motivator in humans — even in very young babies according to evolutionary biologist David Haig, who says babies may cry in order to prevent their parents from having another baby who will need to share their resources and attention. Haig, who discusses his theory in Evolution, Medicine and Public Health, notes that babies obviously can’t force their parents to use birth control, so they use their not-so-quiet voice to force parents into a birth control situation, by waking mothers up to breastfeed. Frequent breastfeeding, though not a perfect form of birth control by any means, may indeed prevent a subsequent pregnancy. Not to mention that waking parents up at all hours of the night enforces an ongoing state of libido-killing fatigue which can kill off the desire for another baby in just about anyone.

Oddly, Haig points out to Science News that breastfed babies seem to have impeccable timing too, as they ramp up their “nighttime breast milk 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.” Now, it’s not like your little bundle is trying to be evil. As Haig says, your baby isn’t intentionally trying to be an only child forever and he isn’t trying to drive you insane, it’s just that babies who do cry for breast milk at night have a major survival edge over babies who sleep soundly. In not so shocking related news, not everyone agrees with Haig’s theory. Other scientists and parents have pointed out that babies cry for multiple reasons, such as being cold, hot, hungry, bored, sick, needing attention, hating their socks and as most parents soon learn, a BILLION other reasons. Past research also shows that a parent rustling around nearby may wake a baby. Sadly, we’ll probably never know exactly why babies have evolved their little selves to cry at night, but if you’re just plain worn out, try these methods to get your little screamer to sleep.

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+ Troubled sleep: Night waking, breastfeeding and parent–offspring conflict

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