But according to new research at Harvard, manipulative babies have another reason for their crocodile tears: They want to prevent their parents from having sex.

According to David Haig, an evolutionary biologist at Harvard, babies don’t want additional siblings competing for their parents’ love. “I’m just suggesting that offspring have evolved to use waking up mothers and suckling more intensely to delay the birth of another sibling,” said Haig. He encourages parents to train their babies to sleep through the night.