At some point, most parents who move on to having a second — or third, or fourth — child see their older child get jealous as the amount of attention they once got shifts toward the newborn. But new research published in the journal Evolution, Medicine, and Public Health finds that these behaviors may be based on more than jealousy, and that there doesn’t even have to be a second child in the house. In an only child, the same behaviors can be seen when they’re crying at night.

In an evolutionary twist most of us didn’t see coming (Most babies that cry at night are just hungry, right?), Harvard evolutionary biologist David Haig suggests that babies who wake up in the middle of the night to get breastfed are really crying so that they can distract their mothers from any late night rendezvous that may result in them having a sibling. But the reason for this might not be out of selfishness.

“Short delays until the birth of a younger [sibling] are associated with increased mortality in infants and toddlers, especially in environments of resource scarcity and rampant infectious disease,” Haig wrote in the journal

Considering that most babies start to cry at night sometime within the six- to 12-month-old mark, it makes sense, as these are some of the crucial moments for nurturing as well as the time (nine months later) that a sibling could come into being. It’s an especially genius inherited behavior too, because the child not only distracts his mother, but also deems her infertile, if only for a little while through lactational amenorrhea, the process by which breastfeeding delays menstruation. Supporting the theory, Haig points to bottle-fed and infants who’ve stopped breastfeeding, who also tend to wake less often during the night.

Yet, according to Holly Dunsworth, an anthropologist at the University of Rhode Island, nightly breastfeeding is more than just about feeding, she told NPR. “There are so many good juices running through infant and mom,” she said. “It’s rewarding beyond the calories and hunger satiation for everyone involved.” She says that it’s also about love and warmth, and that “When you look at it from that perspective, waking up to feed looks more like cooperation than conflict.”

Meanwhile, other experts’ written responses argued that babies may just wake to the sound of their mothers or as a way to prevent sleeping too deeply, which could be dangerous. Another argument against the theory is that babies’ wakefulness comes from a time when the entire population’s sleep patterns were more fragmented, Science News reported.

Although Haig’s work is based on a 1987 paper, “A Suggested Adaptive Value of Toddler Night Waking: Delaying the Birth of the Next Sibling,” many parents should probably go with their gut feeling as well as the advice of their child’s pediatrician.       

Source: Haig D. Troubled sleep: Night waking, breastfeeding and parent–offspring conflict. Evolution, Medicine, and Public Health. 2014.