LOVE STORY | BIRTHDAY | FICTION
The Birthday Paradox
A Short Love Story (umm, well, kind of)
Cheng and Meilin were enrolled in all the same classes the fall of their first year in med school at China’s Zhejiang University. But they didn’t meet each other until a biostatistics class they took together the following spring.
Biostats wasn’t very popular among first year students. Too boring. But, their professor, Dr. Wu, sought a way to make it more interesting. He decided the “birthday paradox” would be an exciting way to engage the students. The birthday paradox is a phenomenon in probability that states that in a group of 23 people, there is a 50% chance that at least two will share the same birthday. It’s not clear why they call it a “paradox.” A paradox is supposed to be a statement which seemingly contradicts itself.
Paradox or not, it seemed counterintuitive to everyone in the class, as each student expected a much larger group would be needed for such a high likelihood of shared birthdays. After getting their attention, Dr. Wu went on to show them how the math behind it all worked.
“In a class of 25 students such as this one, the likelihood is even higher. Better than 50–50!”
To demonstrate it in action, Dr. Wu asked each student to write down their birthday on a sheet of paper and pass it in. One by one, he wrote each student’s birthday on the chalk board, looking for duplicates. Sure enough, there was one pair of students sharing the same birthday.
It was Cheng and Meilin.
“Amazing,” their classmates all gasped as if they were watching the Houdini of the East.
Meilin and Cheng glanced at each other after the discovery and smiled. It felt kind of special sharing something as personal as a birthday. After class, Meilin took the lead in making a more formal introduction to Cheng. A short conversation on their way to the next class led to a study date that afternoon. “I’ll bring tea for us,” Cheng promised, asking Meilin how she liked hers. “Brewed green tea, with honey and lemon, thank you,” was her reply. It was the exact order he usually placed for himself.
By Saturday, a couple of study dates in the student union had led to a lunch date at The Lazy Panda, where many of the other med students hung out. Their classmates looked at Cheng and Meilin and commented on what a terrific couple they made, surprised to learn the pair first met only a few days before. “You look like you were made for each other,” one student told them.
“A perfect match!”
The relationship continued to develop. Meilin and Cheng discovered they had much in common — from the sports they played growing up, to classes they liked and didn’t like, to movies and television shows they appreciated. Similar tastes in music helped them deepen their bond. When Cheng discovered that Meilin loved the band, Dream Garden, he began to think she might be the one for him. They had been too young to have seen the band live back in the eighties, but they enjoyed going to see tribute bands whenever they played at The Lotus. And they loved singing “Love From Afar” when it played on oldies radio.
By the end of their second year, the relationship had blossomed like a Cherry tree. The couple became engaged and before they started their medical residency, they tied the knot.
All thanks in part to the serendipitous discovery that they shared the same birthday! What’s the probability of that?
The early part of their marriage only proved what a perfect match they were for each other. They were both morning people, liked similar foods, and loved sharing embarrassing and intimate things about themselves. Meilin wasn’t afraid to show how she could burp the Chinese National Anthem. Cheng wasn’t afraid to show how he could perform the same song by cupping his hand under his arm pit. “Never tell anyone else I did that!” each said, giggling sweetly.
The couple also had inside jokes they used to assign numbers to. They could make each other laugh just by referencing the number. They finished each other’s sentences. They had a private language that only the two of them understood. What a blessing they were to each other.
When it came time to talk about having kids, they both wanted to be parents, but Meilin wasn’t quite as sure as Cheng was.
“I was adopted,” Meilin said, “and that makes me more cautious about children.” She knew nothing about her birth family or her family’s medical history. She didn’t know what health issues might await her when she got older. “Before becoming a mom, I want to make sure I am not pre-disposed to getting a disease that might kill me while our child is young. I know what it’s like to feel abandoned by a birth parent. I never want my child to feel that way.” It didn’t matter that the reason she was put up for adoption was driven by the country’s “only one child per family” rule at the time. “If I’m going to have a baby, I want to make sure I will be around to raise it.”
Cheng was supportive of Meilin here and suggested they both undergo genetic testing to discover what predispositions existed on their DNA. It might help them in making decisions about having kids. Cheng knew, for instance, he had an aunt with Cystic Fibrosis, a chronic lung disease that worsens as one ages and often leads to pre-mature death. It might be good to know if Meilin also carried the gene.
One of the other things they had learned in their biostats class was how the likelihood of a baby being born with a disease can increase dramatically when both parents have the same genetic marker. For example, while the likelihood of a child being born with Cystic Fibrosis was only 1 in 6,000, that likelihood increased to 1 in 4 when both parents carried the gene.
It was a scary proposition!
Cheng and Meilin enlisted the services of a doctor at their hospital to administer the test. It might have been faster to use a direct-to-consumer testing company like the “23&Me” of China, but they decided to use a professional, both to make sure they were tested for the right things and to interpret the results in the right way. After the pair spit into a vial, their doctor sent the samples to a lab along with instructions.
A couple of weeks later, the doctor invited Meilin and Cheng back to her office to talk about the results. “Sit down, please,” she said professionally, trying her best to make some small talk about the weather. But something didn’t seem right. She was nervous and her brow was furrowed even though she tried to smile.
“I don’t know how else to tell you this, except to come right out and say it, but you do both share the gene for Cystic Fibrosis. Obviously, you don’t have the disease yourselves, but because you both carry it, the probability of having a child with it is quite high.”
“Wow,” Cheng said, shocked to hear the news. “It’s a good thing we checked,” Meilin agreed. “Perhaps we should consider adopting rather than having children together biologically.”
But the doctor wasn’t done. She asked them to remain seated, so she could go through the results of all the other tests she had ordered.
“Thalassemia? Cheng yes. Meilin yes.”
“Hemophilia? Cheng no. Meilin no.”
“Congenital Adrenal Hyperplasia? Cheng yes. Meilin yes.
The doctor continued through a long list of the tests she had ordered. For every genetic marker tested, the doctor confirmed that the results showed that the two were a perfect match.
Every single trait.
“What is the probability that two people share the exact same genetic traits the way we do?” Meilin asked the doctor smiling, while winking at Cheng. “We really do have a one-in-a-billion type of love connection,” she said to Cheng, making him smile.
But the actual probability was somewhat different from what Cheng and Meilin had expected.
“If the pair is unrelated, the chances are mathematically zero” she said confidently. “But, if the pair is related, the chances of such a match are quite high. One way this can happen is if you are biological siblings. And based on your identical birthdays and the test matches, I can only conclude that you are twins!”
Not just husband and wife, but brother and sister, they realized in horror, scooting their chairs away from each other, with an uncomfortable look.
Because they were a perfect match, they were no longer . . . well . . . they were not a perfect match.
Quite a paradox.
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