Exactly four years after a British mother gave birth to a pair of twin boys, on July 18, she delivered a second set of naturally conceived twins, according to a report Thursday in the British newspaper The Sun. The mom, Kim Hefer, is reportedly the first woman in the United Kingdom to have to two sets of twins on the same day years apart. A bookie set the odds of this happening at 30 million to one, the paper reported.

That’s only the most recent case of attention-grabbing coincidental births. A mother, daughter, and granddaughter all sharing the same birthday or twin sisters giving birth on the same day or a boy and girl born on the same day in the same hospital who grow up and decide to get married -- these odds-defying stories capture our imagination.

What is it about shared birthdays that are so irresistible?

Humans understand the world exists in certain ways; for example, we know that weather patterns, not angry gods, cause thunder. But something we perceive as coincidental, say a mother, daughter, and granddaughter sharing the same date of birth, seems so random to us so we believe that supernatural forces cause this coincidence.  

“Basically any event is unlikely. If you flip a coin five times and it comes up heads each time, you think that something funny [has happened]. The probability of getting heads, heads, heads, heads, heads, is the same as any other sequence. It’s not that it is unlikely … there is something else about it that strikes us,” says Tom Griffiths, director of the Computational Cognitive Science Lab and the Institute of Cognitive Brain Sciences at the University of California, Berkeley.

Griffiths and his colleague wrote a paper, which explores coincidences and also considers “the birthday problem.” A classic statistical exercise, the birthday problem aimed to discover how many people needed to gather in a room for two to share a birthday. If there are only 23 people in a room, there is a 50 percent chance that two share a birthday and this likelihood increases as the number of pairs entering the room grows. Griffiths’ findings reinforce what experts know about the birthday problem—shared birthdays aren’t as random as we like to think.

“Why is it that people having the same birthday is something that strikes us? Is it that it suggests there is some underlying reasoning behind it? Our brains want to believe that it is something other than chance,” Griffiths says.

Atlanta psychologist Robert Simmermon agrees that people assign meaning to random events, believing God, fate, karma, or the stars contribute to chance.      

“One in 30 million chances … that chance is incomprehensible,” says Simmermon, who has a private practice. “We really can’t understand anything else that might be like it.”

And people feel more intrigued by these birthday coincidences because birthdays carry a lot of emotional weight.

“It’s the genesis; it is the beginning,” he says. “The birthday is the beginning of our existence … well, the beginning of our consciousness.”

Do you share the same birthday as a sibling or other close family member? Share your unusual birthdate coincidences on Facebook

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