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Welcome. I came here today to warn you

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about the dangers of ice-cream.

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You may not be aware of this,

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but these innocent looking cones full of sweetness

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are one of the major causes of drownings.

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And I've got the numbers to prove it.

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So, if you plot a graph of the number of ice-creams that are sold,

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and you compare it with the number of drownings,

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you can see there is clearly an upwards trend,

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and I think it's very safe to conclude from this

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that we should ban ice-cream because it's very dangerous.

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(Laughter) (Applause)

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Since you're all smart people,

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you've probably figured out

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there's something wrong with my example.

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What's really happening here is, of course,

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that there is an underlying factor,

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which is nice weather, you might have guessed it.

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And if the weather is nice,

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more people will go out swimming,

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and unfortunately drown, and at the same time,

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more people will buy ice-cream.

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And it's not the ice-cream that's causing the drownings.

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And here it's really easy to see that

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there is something wrong,

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but jumping to an incorrect conclusion about causality

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when you see a correlation

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is the most often made logical mistake.

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And today my goal is to make sure that

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in the future you can recognize this mistake.

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And I really hope you can avoid making it

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in the future for yourselves.

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And I'll do this by just giving some famous examples.

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And the first one is really rather innocent.

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The fact is that

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married men live longer than single men.

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If you look at the statistics, you see that

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this is really happening.

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And women's magazines, they like to conclude from this

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that marriage is very healthy for men,

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because it makes them live longer.

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Well, a friend of mine, he likes to joke that

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marriage mainly makes life seem longer,

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but -- (Laughter) --

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that's because his wife is --

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(Cheers) (Applause)

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But so, can anyone guess what's going on here?

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Because there is a causal relation,

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but it's the other way around.

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The fact is that men who are healthy,

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and rich, and well educated,

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and have a much higher life expectancy,

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these are the men that are much more likely

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to find a wife -- that's the way women are --

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and the guys who have a very low life expectancy,

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so they're unhealthy and poor,

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they are not as likely to get married.

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So it's the high life expectancy

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that is causing the marriage,

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not the other way around.

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Well, and this, of course, you know,

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it's not so serious, no one will get married

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just because he read this.

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So let's move to a more serious example.

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It was also more serious research.

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In Nature there was a study in 1999 that showed

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that young kids who sleep with the lights on,

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that they have a much higher probability

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of becoming short-sighted later in life.

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But the researchers, they were smart,

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and they wrote very careful

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that they had found a correlation,

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and they didn't know how the causal relation might work,

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but just to be sure they advised all parents

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to turn off the lights at night.

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And in the popular media this became that

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bed lamps were night abuse, children's abuse,

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and that it was very bad

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if parents used lamps in the bedroom.

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And many parents were worried.

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I can imagine, if this would have happened

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when my son was sleeping with the lights on,

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I would had felt really bad.

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But luckily, the article had to be corrected

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the next week, and maybe some of you can guess,

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and if there are biologists in the audience, they know.

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Short-sightedness is genetic.

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And so, it's parents who are short-sighted.

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And those are the parents who like to

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leave the light on in the bedroom,

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and they also are the parents

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who have short-sighted kids.

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So again, a simple mistake, easy to make.

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Then, what is I think the worst example I know --

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I know many of them,

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I see at least one of these

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in the newspapers every week.

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But this is a classic one: in the '70s,

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researchers found that there is a very strong link

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between kids who do well in school, get good grades,

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and kids who have a high self-esteem.

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And they concluded from this

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that it's very important to make sure that

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young kids are, you know, raised

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to be confident and proud of themselves,

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because if their self-esteem is high,

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the good results will follow.

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And this forethought was told to parents,

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especially in the US, for generations,

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that just make sure that your kid is proud and confident,

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then all will turn out well.

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And many years later, someone did another study

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just to see in which

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direction the cause was working.

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And they found that it was in the opposite direction.

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So the good grades were causing the self-esteem,

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and self-esteem wasn't causing good grades.

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And it was even worse.

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So kids who are raised just to have high self-confidence,

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and not excel at anything --

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it can be sports or music,

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doesn't have to be school --

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the kids who are just proud of themselves,

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and then fail at everything,

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in the end they will have a very low self-esteem,

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and not be able to make anything of their life.

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So this was a very serious correlation mistake.

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And what I want for today is for you

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to remember that the next time

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someone wants to prove that there is a causal relation

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between something and something else --

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it can be anything, it can be vaccines and autism,

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it can be female bankers and the financial crisis,

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and if they -- (Laughter) -- point out to you

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that there is a very strong relation,

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remember that it's not enough to have a correlation.

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It gives a very good hint of what might be happening,

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but before you can conclude that one thing causes something else,

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you need to know why it does and how it does.

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So, when in doubt, just remember the ice-cream.

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Thank you very much.

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(Applause)