When pregnant best friends share similar due dates, it’s offhandedly attributed to coincidence or “something in the water,” but there’s actually a strong sociological explanation for syncing up fertility decisions. With word of a possible fourth Kardashian baby on the way—Kim Kardashian West and Kanye West are expecting via a surrogate, Khloé Kardashian and Kylie Jenner are reportedly in the early stages of pregnancy, and unconfirmed rumors now suggest Kourtney Kardashian may also be expecting—the close-knit sisters may be exercising a primal urge toward strength in numbers by seeking out the shared experience of delivering babies mere months apart.

A study appearing in the American Sociological Review suggests the desire for kids can have a virus-like effect among friends. As one woman in a tight circle decides to try to get pregnant, those on her speed dial are likely to consider joining her. Victoria’s Secret Angels Behati Prinsloo and Candice Swanepoel, followed by Rosie Huntington-Whiteley, are an example of friends who recently synced pregnancies. Longtime confidantes Naomi Watts and Nicole Kidman also have children close in age. As for the phenomenon pre-dating Hollywood? There are even biblical references to Elizabeth and Mary, the mothers of John the Baptist and Jesus, giving birth just months apart. So what are the psychological factors driving women’s desire to pair up their pregnancies?

UCLA Psychology Clinic director Danielle Keenan-Miller, Ph.D., says basic survival instincts play a major role in fertility decisions among friends, though there is also a practical explanation. “Friends tend to be around the same age and reach milestones like jobs and marriage at nearly the same time,” she says, explaining they’re also likely to consider pregnancy in a similar time frame. But beyond the basics, there are three types of conscious or unconscious thoughts prompting friends to band together for reproduction.

First, Keenan-Miller explains, social learning about parenthood among groups of friends sparks an interest in babies. We consider parenting while watching friends and relatives do it. “It can be as simple as realizing, ‘I love my cousin’s child or my best friend’s child, so maybe I can be a parent, too,’” Keenan-Miller says. Second, there’s a ready-made social support network created when women have children simultaneously with others in their circle. “Parenthood can be daunting, and when people go through something that makes them afraid, there’s the perception that it’ll be better if there’s someone out there who is also going through it [and] can hold your hand.” The third factor is plain social pressure. “Evolutionarily, our survival depended on being part of a group,” explains Keenan-Miller. “Fear of being excluded and lonely or unsupported can also play a role.” Social pressure, she adds, can be particularly grating for families with fertility challenges and those who don’t want kids.

Taking a joint plunge into motherhood, Keenan-Miller points out, sets up an instant support network that can boost mood, calm anxiety, and foster a sense of community. For most, forming a baby group with your besties is just an added benefit. As for the Kardashians? It could also mean another decade run of Keeping Up With the Kardashians—The Next Generation.