Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

2. Reframe the question. Is that soap 99.4 percent pure or 0.6 percent toxic? Politicians, advertisers, and even our local supermarket staff routinely spin just about everything we hear, see, and read. Everything is presented to be as positive as possible. Our job — as consumers, voters, and citizens — must be to perpetually cast a skeptical eye and develop a habit of rethinking whatever we are asked. (Should I construe this "assisted suicide" legislation as an effort to protect people from murderous doctors or as a way of allowing folks to die with dignity? Should I think about the possibility of reducing my hours to part-time work as a pay cut or as an opportunity to spend more time with my kids?) If there's another way to think about a problem, do it. Contextual memory means that we are always swimming upstream: how we think about a question invariably shapes what we remember, and what we remember affects the answers we reach. Asking every question in more than one way is a powerful way to counter that bias.

3. Always remember that correlation does not entail causation. Believe it or not, if we look across the population of the United States, shoe size is highly correlated with general knowledge; people with bigger shoes tend to know more history and more geography than people with smaller shoes. But that doesn't mean buying bigger shoes will make you smarter, or even that having big feet makes you smart. This correlation, like so many others, seems more important than it really is because we have a natural tendency to confuse correlation with causation. The correlation I described is real, but the natural inference — that one factor must be causing the other — doesn't follow. In this example, the reason that the correlation holds is that the people with the littlest feet (and tiniest shoes) are our planet's newest visitors: infants and toddlers, human beings too young to have yet taken their first history class. We learn as we grow, but that doesn't mean that growing (per se) makes us learn.*

*Pop quiz: should we study the dictionary to make ourselves smarter? Maybe, maybe not: lots of websites promising to build vocabulary tell us that "people with bigger vocabularies are more successful," but is it the vocabulary that makes them successful, or some third factor, like intelligence or dedication, that leads to both success and a large vocabulary?

4. Never forget the size of your sample. From medicine to baseball statistics, people often fail to take into account the amount of data they've used in drawing their conclusions. Any single event may be random, but recurrence of the same pattern over and again is less likely to be an accident. Mathematically speaking, the bigger the sample, the more reliable the estimate. That's why, on average, a poll of 2,000 people is a lot more reliable than a poll of 200 people, and seeing someone bat .400 (successfully getting a hit in 40 percent of their tries) over 10 baseball games doesn't mean nearly as much as seeing them bat .400 over a 162-game season.

As obvious as this fact is, it's easy to forget. The person who first formalized this notion, known as the law of large numbers, thought it was so obvious that "even the stupidest man knows [it] by some instinct of nature," yet people routinely ignore it. We can't help but search for "explanations" of patterns in our data, even in small samples (say, a handful of baseball games or a single day's stock market results) that may well reflect nothing more than random chance. Boomer hit .400 in the last ten games because "he's seeing the ball real well," never because (statistically speaking) a .300 hitter is likely to occasionally look like a .400 hitter for a few days. Stock market analysts do the same thing, tying every day's market moves to some particular fact of the news. "The market went up today because Acme Federated reported unexpectedly high fourth-quarter results." When was the last time you heard any analyst say "Actually, today's rise in the market was probably nothing more than a random fluctuation"?

Happily, psychologist Richard Nisbett has shown that ordinary folks can be taught to be more sensitive to the law of large numbers in less than half an hour.