For a long time, mothers have been pushed this narrative that the ideal spacing between siblings is two years.
So far, no one has questioned this. The belief is that siblings spaced this way will enhance their bond and it’s better to ‘get it over and done with’. But evolution suggests otherwise.
Science writer and consultant Elena Bridgers (@elena.bridgers) took to Instagram to debunk the two-year ideal, explaining that, for most of human history, the natural spacing between siblings was actually closer to four years.
Bridgers’ insights—rooted in the concept of evolutionary mismatch—challenge the pressure many mothers feel today to have their children in rapid succession.
“There are so many silly reasons that people give to pressure mothers into having their kids close in age, but no one is talking about how much this increases parenting stress,” the 36-year-old said.
Raising two young children at the same time—especially without substantial help—can be exhausting. And contrary to popular belief, siblings spaced more closely actually tend to fight more, not less.
While two-year spacing may feel “normal” today, research on hunter–gatherer societies suggests that the natural interbirth interval is actually about four years.
“Since all humans lived as hunter-gatherers for 95 percent of our evolutionary history, it’s reasonable to assume that this was the case for all humans,” Bridgers said.
The support for this is also in primatology: “Chimpanzees, our closest living relatives, have a six-year interbirth interval and orangutans have an eight-year interbirth interval,” she added.
A key question is why did our ancestors have children further apart?
It largely came down to breastfeeding, diet and lifestyle. In traditional societies, mothers breastfed on demand for about three years, which naturally suppressed ovulation.
Additionally, they had lower body fat and more physically active lives, both of which reduced fertility.
Today, modern factors like formula feeding, sedentary lifestyles and high-calorie diets have shortened the interval between births, but this gap doesn’t make it ideal for parents’ wellbeing.
Bridgers shared that raising two children under two with very little help was tough.
“When you have a newborn and a toddler in the midst of the terrible twos, it’s a whole different ball game, and honestly, it’s not great,” she said. “It measurably increases parenting stress which is positively and robustly correlated with postpartum depression.”
A smaller age gap also intensifies sibling rivalry, as children close in age tend to compete more for parental attention. In contrast, larger age gaps are correlated with better sibling relationships and less competition.
Bridgers told Newsweek that the reaction to her Instagram reel has been overwhelmingly positive.
“Moms are very interested in this research because I think they find it validating,” she said.
She explained how women in all different stages of motherhood could relate. Mothers who had kids in quick succession, like her, validated why their experience was challenging.
“The mothers who only have one and are planning a second, and whose instinct is telling them to wait, feel reinforced. Even the ones who planned their families so that their children would be close in age, and who are happy with the outcome, [found] it interesting.”
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