Previous: Accio Decade Hallows
Next: What OCD Is Like (for Me)



View count:799,114
Last sync:2023-05-10 23:15
I have always thought that sex ed and statistics should be the same course. There's no better way to show how vital stats is to your every day life than to look at statistics. Probabilities are so easy to misunderstand, and there are a lot that go into understanding the outcomes of sexual activity.

There's a ton I didn't get into that a common trap is that, in a pinch, one might have unprotected sex with no negative outcome, and that makes it /feel/ much safer for future times. Which, I think, is a common trap people fall into.

There's also the fact that a 2% chance sounds pretty tiny, but it really does mean that, on average, if you have 50 couples using condoms perfectly, one of them is going to get pregnant in any given year. That's gonna seem weird and surprising and improbable and unfair to that couple, but statistically, it's exactly what we expected. Probabilities eventually collapse into reality, and then that couple no longer has a 2% chance of probability, they have a 100% chance, because they're pregnant, and everyone else has a 0% chance.

Being one of the two percent isn't that weird.

Here's the video I made on probability in 2010 that I'm actually pretty proud of:

Most of the information used in this video came from the AMAZING AND WONDERFUL Guttmacher institute:

Contraceptive Use in the US:

Unintended Pregnancy Rates:

IUDs Cause Decrease in Unintended Pregnancies:

Subscribe to our newsletter!
And join the community at
Help transcribe videos -
John's twitter -
John's tumblr -
Hank's twitter -
Hank's tumblr -

Preorder John's new book, Turtles All the Way Down, out October 10th 2017! You can find links to both the signed and unsigned editions here: and information on how to (probably) get a signed copy here:

Good morning John, so, this video is going to be about birth control and statistics, so if you're not super down talking about sex, or math, or you get the wibblies thinking about me talking about either of those things, I don't mind if you check out on this one.

For those of you who stayed, I remember when I first took sex ed, I was told that condoms had a 2% failure rate. And there's about 25 different ways that you can misunderstand that statistic, so I want to talk about that.

First, what kind of failure, 'cause condoms do more than one thing. They reduce the chance of pregnancy but also of disease transmission. Or when they say failure do they mean, like, a structural failure of the condom, which may or may not result in pregnancy or disease transmission. Well, in this case what my sex ed teacher was talking about and what we're talking about in this video is, unwanted pregnancy.

So my teacher was saying when using a condom perfectly and properly for vaginal sex, there is a 2% chance of pregnancy occurring. But, like, per what? I thought, at the time, that it was per sex. That makes a certain amount of sense. So like, every time you do it, you have a 2% chance of pregnancy. And that would be pretty terrible, like that would be just 10 times better than just having unprotected sex. But whenever you read about unintended pregnancy rates with birth control, that's per year of use, not per individual intercourse. So over a year of perfect, proper condom use, there is a 2% chance that a couple will get pregnant. Not a couple, just the one person. But y'know.

This actually, for me, is a little less good, because one sexually active person might have way--like 10 times more sex than another sexually active person. And the person who has more sex will be more likely to get pregnant using only condoms. But that's how it gets measured, because for the most part these statistics aren't meant to help individuals make individual decisions. They're meant to inform public health decisions by policymakers. But they get used by individuals because they're the only stats we have.

Per act of intercourse, the odds of pregnancy decrease about 250 times if you're using a condom. That's a lot better than just 10 times. And that is a statistic that, as far as I can tell, has never been taught in a sex ed course because I had to calculate it myself using somewhat incomplete raw data.

Now, in addition to misunderstanding what the 2% statistic meant, I also misunderstood how statistics work, and I thought that meant that if there was a 2% chance of unintended pregnancy then if you did it 100 times, there would be 2 pregnancies. That's not how probabilities work. You have a 2% chance every year. So you have a 2% chance the first year, a 2% chance the tenth year, a 2% chance the 100th year, though, not actually. 100th year seems unlikely.

Every year, it's a new roll of the dice. But statistics does allow us to measure probabilities for multiple rolls of the dice. So, if you go for two years, you multiply 0.98 by 0.98 to get your odds of not getting pregnant. If you use condoms perfectly every time for ten years, your odds become about 20% that you will get pregnant, and over 20 years that becomes one in three.

Now the final, and vital, way that this statistic is misunderstood is that perfect use is very different from typical use. And when we measure typical use we see that condoms result in pregnancy about 20% of years in which people only use condoms. That is a very different number from 2%. Typical use is how the average person uses birth control, and you may not be an average person, nobody really is, but if there's a way to mess up, people often find that way.

Which is why new forms of birth control, like implants or IUDs, if they work for that particular person, have had a really substantial impact on unwanted pregnancies in the world and in America. These technologies' typical use is the exact same as their perfect use, because they're just always doing the thing they're supposed to do. And they're also even more effective than perfect use of condoms. Though, neither of them, to be clear, protect against diseases, obviously.

John, I made this video because I was really confused when I was given these statistics. I wish that I had had more than the basketball coach begrudgingly parroting some politicized curriculum at me. Hopefully, the existence of the internet and continuing work of scientists to understand this stuff will allow people to get better information and make better decisions even as people continue to fight to hide this stuff.

John, I'll see you on Tuesday.