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Hey there, it’s Stephen Dubner. Earlier this year, we added a new show to the Freakonomics Radio family. It’s called Freakonomics, M.D., and it’s hosted by Bapu Jena; he is a medical doctor and an economist who does a lot of fascinating research, so you can see why that’s a good fit for us. You can get Freakonomics, M.D. wherever you get Freakonomics Radio, and I’d suggest you go do that right now. Just in case you need a little convincing — today, I’d like to play you an episode of Freakonomics, M.D. This one is about Covid-19 and what you might call the birthday effect. It’s based on a paper that Bapu co-authored and was published by JAMA Internal Medicine. But before we get to the episode, I wanted to check in with Bapu on a related matter.

Stephen DUBNER: Bapu, how are you today? 

Bapu JENA: I’m doing well. Yourself? 

DUBNER: Good, thanks. I’m calling because I wanted to pick your doctor brain for a minute. I’m sure you’ve got friends and family always asking you medical questions, but if you don’t mind I have one more.

JENA: Can I bill your insurance for this, or no? 


JENA: No? Okay. Go ahead. That’s fine. 

DUBNER: So, here’s my question. Freakonomics Radio traditionally has a holiday party. Last year — first year of the great Covid lockdown — we didn’t have one. This year, we are. A decision was made to have a party — I’ll put it that way, because I was not the deciderator. And I’ll be honest with you, I’m not feeling great about it.

JENA: First of all, why wasn’t I invited to this party?

DUBNER: Were you really not invited to our holiday party?

JENA: No, no. I was invited. I can’t make it, but I was invited.

DUBNER: Now, you can’t make it because you live in Boston, presumably.

JENA: That’s right. We’ve got two young kids, so it’d be hard. I mean, I wish I could be there, but it’s not Covid that’s keeping me from there. It’s really the distance.

DUBNER: So here’s the thing: I wonder if we’re being idiots for having this party, because when you think about risk and reward — obviously there are vaccines, and everyone at this party will be vaccinated. But then I’m concerned that those people — many of them are younger people — will go home to parents and grandparents and I feel deeply conflicted about this, and I wonder if you have any advice for me. 

JENA: Yeah, I understand and I’ll tell you we’re facing the same dilemma ourselves. Our daughter just turned seven this month. And we were deciding whether or not to have a tea party in the house, because she just got vaccinated.

DUBNER: And what did you decide? 

JENA: Um, it was decided — I think the language I would use is “it was decided”.  

DUBNER: So, you and I are basically in a similar situation. We’re getting peer-pressured into hosting and attending a party that we might not be so enthusiastic about. 

JENA: Yeah. What we’re going to do is they’re going to make the tea and prepare some of the stuff inside, and then try to do the eating outside with the mask down. That’s our goal.

DUBNER: So, my concern about our party is it’s being held in a restaurant or bar where it’s inevitably inside. And we’re always making decisions in life. There’s risk, there’s reward. I’m not worried about me getting sick and dying of Covid — although I could. But I am worried about community spread. I do also wonder if I’m falling prey to what people like you call the “recency bias.” I learned that a couple of family friends recently died from Covid — both fairly elderly, but not that old. So do you think the recency of those deaths is maybe skewing my decision-making on this party? 

JENA: Yeah, and I’ll add one more anecdote to that. I’ve heard of holiday parties — work-related, not my own — but others where, everybody is required to be vaccinated and there are still cases that emerged subsequent to that. So, I mean, there is a risk. I think it would be incorrect to say that there’s no risk. 

DUBNER: I guess for me personally, I don’t like parties that much. And so the reward of a party — it doesn’t quite rise to the admittedly small risk.

JENA: I mean you could greet everybody from outdoors. You could be the bouncer basically.

DUBNER: That’s a really good idea. Say more about that.

JENA: Well, first of all, you’d have to put on about 60 pounds. You’d stand outside. And just air shake everybody’s hand as they walk in and you’ll be good.

DUBNER: Nice. I like that idea. You know, I talked to another colleague — very smart woman — I said to her, “My concern is not for my crew per se or for myself, but that if someone goes home to their family for the holidays, the way we saw the big spike last year with Covid-19, and gives it to some parent or grandparent who is maybe unvaccinated and they die, I would feel bad about that.” And this person said to me — excuse my French — she said, “You know what? If they’re not vaccinated, f*ck them.” I was a bit taken aback by that position. But how about you? How do you feel about the “f*ck ‘em” school of public-health policy? 

JENA: I think about it this way. I don’t want people to make bad decisions, unhealthy decisions, if they’re not informed about them or if they don’t understand the issues. And that probably was partly the issue early on in the pandemic where there was questions about the vaccine, how safe was it going to be, how effective would it be. Over time we get to a world where information or lack of information really isn’t the problem, but it’s just a belief system. People have different beliefs. Now, maybe other people will disagree, but I am certainly willing to say that people should have autonomy over their decisions, as long as we’ve done our job of making sure that they’re informed.

DUBNER: So, it sounds like you are suggesting that we have the party and I attend the party. Is that ultimately your friend-slash-doctor suggestion, Bapu?

JENA: Yeah, that’d be my suggestion. I would absolutely go in person if I knew that everybody had been vaccinated. I’d feel even more comfortable if people were testing before.

DUBNER: So we should explore the possibility of administering a rapid test before, yes? 

JENA: Yeah. And they’re really available right now. I mean, people probably have them at home. You could just ask them to do it.

DUBNER: All right. Happy holidays. I wish you were coming to the party!

JENA: I know. I wish I was. 

DUBNER: All right. Hopefully next year. And now, Bapu, we are going to hear an episode of your show, Freakonomics, M.D. This is an update of one of the very first episodes, originally called “Covid and ‘The Birthday Effect.’” 

*      *      *

From the Freakonomics Radio Network, welcome to Freakonomics, M.D. I’m Bapu Jena. I’m a medical doctor, but I’m also an economist. And in each episode, I’ll dissect an interesting question at the sweet spot between health and economics. Today: What birthdays can teach us about how safe it was, at the start of the pandemic, to gather with the people we know and trust.

Like I was telling Stephen, my daughter turned seven this month.

FAMILY SINGING: Happy birthday to you! Yay!

This is her second birthday during the pandemic. That clip of our celebration — that’s from last year, when things were still pretty locked down, and it felt really important to try to make our daughter’s birthday special. But it just didn’t feel safe to have friends and family over. So we did the next best thing.

Tricky TIM: Tricky Tim is what I go by. So that’s the name I’ll stick with. Been doing magic since I was four years old. 

We got a magician named Tricky Tim to perform at a virtual party. Yeah, on Zoom. This guy was a life-saver for us but he was hard to book — he’d been busy! I’m telling you, I’ve never seen my daughter laugh that hard. Ever. I mean like a full-on belly laugh — the kind of laughter where you’re laughing so hard, I’m not even sure that oxygen transfer was happening at that point. So, last year, I learned something new. It’s actually possible for a Zoom party to be fun. The party that we had got me wondering, though, about the party that we didn’t have, one with all our friends celebrating in our house. And I couldn’t help but think that other people out there may have made different choices, celebrated their birthdays in person with their friends and families, even as the world was basically shut down.

Mola LENGHI: New Yorkers here are waking up to new rules.  

Elle THOMAS: You’ll find few people walking down Salt Lake City streets. 

Lori LIGHTFOOT: I have ordered the closure of Chicago’s lakefront.         

While all that was going on, we know that people were still gathering with others in their homes. Not everyone followed social distancing and shelter-in-place orders the same way. And as much as life slowed down, it obviously didn’t come to a screeching halt. Even people who were being pretty careful, like me, still had some interactions with others. Now, it is true that big events like weddings and graduation parties were postponed a lot, in part because people were worried about large, super-spreader events. But a lot of epidemiologists were also worried that a key driver of Covid spread wasn’t these huge gatherings, but small get-togethers. For these kinds of get togethers, many of us probably found ourselves bending the rules just a little bit. Maybe for a special occasion with a few friends or family members. Maybe for a birthday. I set out to work on a research project, along with Chris Whaley, Jonathan Cantor, and Megan Pera to see if we could find a measurable link between birthday parties and Covid.

Here was the idea. If we found an increase in Covid cases following birthdays, that might help us understand the effect of smaller group gatherings of any kind on the spread of the virus. The kinds of gatherings with people that we know and trust — unlike going to a bar or a restaurant, for example. And birthdays were actually ideal for a natural experiment. That’s something I’m always on the lookout for as an economist, by the way. A natural experiment is when something in the world provides us with the kind of randomization that we usually only see in randomized trials. No scientist was going to randomize people to small get-togethers to measure the impact of those get togethers on Covid spread — that wasn’t going to happen. But here’s why birthdays actually serve that role.

First, literally everyone has a birthday. As far as variables go, that’s about as universal as it gets.

Second, birthdays are random. You’re not more likely to be born at a certain time of year if you’re rich or poor, Democrat or Republican, from California or Kentucky.

And third, unlike a wedding or a graduation, your birthdate and the birthdates of your family members are actually listed in some of your health data.

And that birthday data meant we had the chance to crunch some numbers. Some really big numbers. We looked at a sample of nearly 3 million U.S. households with private health insurance. Because these are insurance data, we could see if and when someone received a medical diagnosis of Covid-19. And we could see if that Covid diagnosis came within a couple of weeks of when someone in that family had a birthday. For each week between January and November of 2020, we compared Covid-19 diagnosis rates between households with and without a birthday in the two weeks prior. So, what did we find?

In counties with low Covid rates, we didn’t find any increased rate of infection in the weeks following birthdays. That made sense because overall transmission rates in those counties were low. But then we looked at counties that were Covid hotspots. There, the likelihood of infection in a family actually increased by about 30 percent in the two weeks following a birthday, compared with those households in the same county that didn’t have a birthday during the same two-week window. And that birthday effect was nearly three times larger for households in which a child had a birthday. It jumped from 5.8 cases per 10,000 people to 15.8 cases. As a researcher, with the data that we had available, I can’t tell you exactly why the effect was so much larger for kids’ birthdays. But as a parent, I can make some pretty good guesses. First, it’s hard for parents to cancel a kid’s birthday party, right? It’s an easier thing to cancel for an adult. And another reason kids’ birthday parties might be more of a Covid risk? If you do have an in-person birthday party for a child, it’s going to be tricky to get the kids to socially distance and wear their masks. And when it comes to older relatives who are at higher risk, especially grandparents, it may be hard to keep them away from a kid’s big day.

Now, it’s important to note that we didn’t have information on actual birthday celebrations. Remember, there’s no national database of who physically had a party, and who didn’t. We only knew when a family member in a household had a birthday. But because birthdays are random, the only thing that should really explain why households with a birthday had higher rates of Covid-19 in the weeks following that birthday is that some fraction of those households celebrated with other people. And so how did we make sure that our finding, which was kinda interesting, wasn’t just random statistical chance? And that the birthdays were actually the cause?

We tested our findings by taking the same set of people and assigning each of them a randomly generated date of birth. Basically we picked a date out of a hat for each person, and compared that fake birthday to the dates of any Covid diagnosis in their family. And when we repeated our analysis, we no longer found a birthday effect. There was no increased transmission of Covid in the weeks following the fake birthdays, which made us confident that the birthday effect that we had found was real. Economists call this a falsification test. It’s sort of an important way to kick the tires when you’re crunching data.

We took a closer look at the data in a few other ways too, and this one might surprise you. The political leanings of the county where someone lived didn’t make a difference. The link between birthdays and Covid-19 was the same whether the majority of the people in the county voted for Donald Trump or Hillary Clinton in 2016. It also didn’t seem to matter if there was a shelter-in-place policy in effect. Which makes sense, since these policies would have been kind of toothless to police smaller, private social gatherings. And, for good measure, we looked at the weather, too. We thought that rainy days might drive people indoors more. But turns out that there was no relationship between rainfall during the week of a birthday and Covid diagnoses afterwards.

Just ahead, birthdays weren’t the only way to study the effect of social gatherings on Covid spread. What if you were a college student during Covid, living in a town where your school was part of one of the biggest competitions in college sports?

Ashley O’DONOGHUE: Essentially 64 colleges across the country were having these giant celebrations.

And we hear from public health expert Dr. Vinay Prasad. 

Vinay PRASAD: You have to count on the fact that not everyone’s going to do everything you say. 

*      *      *

We just learned that families in 2020 — this was before the vaccine was introduced — were more likely to experience a Covid case in the weeks following a birthday in the household. If it was a kid’s birthday, the effect was even bigger. Why? Well, it’s likely families still had some sort of gatherings for their kids. Around the time we were working on our paper, another researcher was actually working on a similar idea, which was recently published in JAMA Network Open.

O’DONOGHUE: My name is Ashley O’Donoghue.

Ashley is an economist at the Center for Healthcare Delivery Science at Beth Israel Deaconess Medical Center in Boston. She’s also a researcher at Harvard Medical School. And she also happens to like a particular college sports tournament: March Madness. It’s when 68 college basketball teams compete to be crowned the best in the country — though four teams don’t make it into the main bracket — and fans love trying to figure out who’s going to make it through the rounds. Including Ashley.

O’DONOGHUE: I create a bracket every year with my friends. I’m not very good at it.  

So, just as she did every other year, in March 2021, Ashley made a bracket. And — well, it didn’t turn out the way she had predicted.

O’DONOGHUE: I had Gonzaga beating Illinois in the final, and that is not what happened at all. Illinois ended up losing in, I think, the second round, so my bracket was completely busted.

It turned out that Gonzaga made it to the final, but lost to Baylor, 86 to 70. But what happened to Ashley isn’t uncommon at all. In fact, it’s a pretty big draw of the tournament: it’s characterized by a lot of upsets, making the outcome feel pretty random.

O’DONOGHUE: And I realized this was actually the perfect situation to measure gatherings.

The 2021 tournament was held in Indiana because of Covid-19. The hope was that a central location for the tournament would make certain logistics, like mass testing, easier.

O’DONOGHUE: There were still lots of parties happening on college campuses.

In other words, Ashley had a natural experiment.

O’DONOGHUE: Essentially 64 colleges across the country randomly were having these giant celebrations.

So, Ashley compared changes in Covid cases over time in counties with the 64 colleges that participated in the main March Madness bracket against those that didn’t. What did she find?

O’DONOGHUE: The main finding was that counties that had colleges that participated in March Madness started to see significant increases in their Covid case rate eight days after the final game.

By “after the final game,” Ashley means after a team was eliminated from the tournament, with cases peaking around 24 days after. Overall, Ashley recorded as much as a 22 percent uptick in Covid cases in counties that had colleges participating in the tournament. What I liked about Ashley’s study was that it was clever and important. It reminds us of the power of needing to feel connected to others. Remember, by March of 2021, the lives of college students had been dramatically altered for more than a year. In that way, for people living in towns with colleges competing in the tournament, March Madness was kind of like your kid’s birthday.

O’DONOGHUE: This was probably their first opportunity to really celebrate and be with their friends.

So, birthdays and March Madness — what do we make of all of this? I asked Dr. Vinay Prasad to weigh in.

PRASAD: I’m an associate professor in epidemiology and biostatistics at the University of California, San Francisco. And I’m interested in oncology, medicine, health policy, and pretty much everything in between.

Researching other health research is actually one of his areas of expertise. Some people call that meta-research.

PRASAD: Meta research is really concerned about the scope, the quality, and how research is done, and the incentives around research.

Full disclosure: Vinay is an old medical school classmate of mine. He has been steeped in conversations about public health during the pandemic. One of the things that he’s emphasized are the tradeoffs that every public-health decision leads to, whether it be closing schools, shutting down businesses, or mandating masks or vaccines. And so I asked Vinay what he thought about my finding that Covid cases increased after people got together to celebrate, of all things, their birthdays.

PRASAD: It’s one of those studies that makes you smile because it’s a clever approach to a problem. It was quite a convincing paper to me. There’s a whole host of different take-aways you could have. One could be that people are cheating, people aren’t doing the right thing. The other thing could be: maybe we missed an opportunity for harm-reduction philosophy.

I asked Vinay what our study tells us about human behavior. 

PRASAD: If your goal is to have sort of a sustained and reasonable pandemic response, to some degree, you have to count on the fact that not everyone’s going to do everything you say, not everyone’s going to do everything you wish they did. People are people. 

And Vinay says remembering that simple fact may have improved our response to the pandemic.

PRASAD: I think appreciating that people are primates and primates have needs has to be a part of any sort of public health response. When it comes to important life events, like a child’s birthday, people want to celebrate those events. And so the strict thing is to say, “You can’t do that.” Another strategy might’ve been to say, “Maybe there are ways you can do that more safely.”

So maybe a blanket order to stay at home or not gather, no matter the occasion, just wasn’t realistic. And maybe now that we have the data on how risky even smaller group gatherings can be, that’ll help us make better decisions too. Because I think our study about birthday gatherings and Ashley O’Donoghue’s study about March Madness both teach us that we may not be as good at assessing the risks around us as we might think or hope. Most people would probably think that interacting with others that they don’t know, like in a bar or a restaurant, would pose a greater risk. But small gatherings with people that you know and trust could cause a different problem. We let our guards down a little bit more. I know I certainly have. I mean, I would wear a mask if I went to the store, but if I had a friend come over, would I always have a mask on inside? I tried to, but I don’t know that I always did. Our study also doesn’t speak at all as to what’s good or bad, right or wrong, but it does help us quantify the tradeoffs that exist. And it’s up to all of us to figure out what we do with that better understanding of risk. Speaking of risk, I decided to check back in with Stephen Dubner. I was curious: how did the Freakonomics Radio holiday party turn out?

JENA: Stephen, how’s it going?

DUBNER: Pretty good, Bapu. How are you? 

JENA: I’m good. So, how was the party? I’m sorry I had to miss it.

DUBNER: Well, as it turns out, everybody missed it. We had no party. 

JENA: Okay, this sounds like classic Covid, but I’m curious to hear what happened.

DUBNER: Well, I guess you might assume that we chickened out, even though your advice was take precautions, have everybody tested. But it wasn’t chickening out. We did follow your advice. I was ready to bundle up outside like a bouncer, greet people on the way in, on the way out.

JENA: I told you, that’s a good look for you. You would have been perfect in that role.

DUBNER: I agree. Let me back up a little bit. So the party was timed to coincide with the first day that all the crew on Freakonomics Radio were working in-person together for the first time in nearly two years. And in fact, we brought in most of the people who work out of town — because during Covid we did a lot of hiring and many people didn’t work in New York. So we actually had what we were calling a “summit,” which in retrospect — I don’t know if “idiotic” is the right word, “silly,” “optimistic,” some combination of those? But we had us a summit. Like Vinay Prasad said, “We are primates, and primates have needs.” But then we did some testing a few hours before the party. And lo and behold, I believe it was one of the first people who was tested — bingo! Covid.

JENA: It sounds like a peak Covid summit. It’s not the kind of summit that we want.

DUBNER: And so thanks to you, because we did test before, we shut things down. This poor person felt terrible. This person is still asymptomatic as far as I know, so that’s the good news. We did have a second person test positive a couple days later, and that person is also okay. But you potentially saved us from having our own little Freakonomics super-spreader event. So I guess that outcome is better than it could have been.

JENA: Wow, that’s crazy. I’m surprised. For Thanksgiving, we had my parents come, and they’re older, my in-laws, family friend, and we had everybody tested twice. They tested before they left their town, and then I tested them at the airport as I was picking them up — and everybody tested negative. But we took those precautions. I think the world is different now than it used to be and we have the ability to test now. So I’m grateful that you did it.

DUBNER: I’m really grateful too, plus, which, as I told you, I don’t like parties that much. So, I hate that it took Covid to get out of the party, but I got out of the party. It was also interesting to note that because people were not so accustomed to being back in a place like the office, there was no set norm. Some people were masked in the beginning and some were not. And then some adjusted to the others in one direction and then some adjusted to the others in the other direction. And I found that we’re still often in this kind of purgatory where no one quite knows what to do fully, no one knows quite how to behave or what to suspect or who to suspect fully. And so at the end of the day, an actual test from a drugstore turned out to be a really nice yardstick that everybody believed.

JENA: The power of facts.

DUBNER: So Bapu, because of my potential exposure to this person, I’m once again quarantining, sitting at home, isolated. I am taking rapid tests and so far I’m negative. But I’ve got plenty of time on my hands and you are continuing to put out these great episodes of Freakonomics M.D. Can you give me a little sneak preview of your favorite upcoming episodes?

JENA: The two episodes that are coming up pretty soon, one is looking at the impact of retirement on your brain. And the basic idea is if you stop working, you stop doing what you’ve been doing for many, many years, using your brain in the way that you have at work — what happens when that switch turns off one day? What happens to your brain? I won’t spill the beans here, but it’ll be a fun episode.

DUBNER: I’m guessing it’s not good news. I’m guessing you’re saying I should not quit my job anytime soon.

JENA: You should never quit. But that’s mostly for the greater societal good that you create as opposed to for your own health. And then the other one which is going to be really interesting is looking at the politics of medicine, really in two ways: one is to figure out how politics affects the decisions that doctors make when they care for patients. We take an oath to make decisions that are in the best interests of patients, but sometimes it’s hard for us to not have our own belief structures, preferences, ideologies impact that care. And then the reverse question is whether or not medicine affects our politics, and I’m going to talk to former U.S. Senator Bill Frist about that. He’s the quintessential example of someone who was a doctor first and went into politics.

DUBNER: Wow, both those sound really good. I look forward to hearing those. I have to pass on some really positive feedback from a friend of mine who’s been listening to your show, who said that there’s so much conversation going around and so much opining about things that we used to talk about in more factual terms and now we talk about them a little bit more like, “this is what I believe to be true.” And he said that it’s hard to just ascertain basic facts sometimes. Of course, with Freakonomics Radio, it is sort of an exercise in fact-finding and putting those facts in context. But he said that he thought Freakonomics, M.D. did an extraordinary job of that. And it’s not just saying, “This is true, this is not true.” It’s the way that you show your homework. It’s the way that you explain why one might think a certain thing is true, but when you surround it with evidence and analysis, you can get a little closer to the truth, and that the big advantage is letting the listener in on the process of learning what is most likely to be true. And he just found that incredibly useful and inspiring. So, I just wanted to pass that along and say thanks, keep up the good work, and I’m really glad and proud that you’re part of this little Freakonomics Radio Network.

JENA: Thanks, Stephen, I’m glad to be here!

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Freakonomics Radio is produced by Stitcher and Renbud Radio. This episode was produced by Tricia Bobeda and Mary Diduch. It was mixed by Adam Yoffe and Eleanor Osborne. Tracey Samuelson was the supervising producer. Our staff also includes Alison CraiglowGreg Rippin, Zack Lapinski, Rebecca Lee Douglas, Morgan Levey, Ryan Kelley, Jasmin Klinger, Emma Tyrrell, Lyric Bowditch, and Jacob Clemente. The music for Freakonomics, M.D. and Freakonomics Radio was composed by Luis Guerra. You can follow Freakonomics Radio on Apple PodcastsSpotifyStitcher, or wherever you get your podcasts.

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  • Tricky Tim, magician.
  • Ashley O’Donoghue, economist at the Center for Healthcare Delivery Science at Beth Israel Deaconess Medical Center in Boston and researcher at Harvard Medical School.
  • Vinay Prasad, professor of epidemiology and biostatistics at the University of California, San Francisco.



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