It's tempting to think of knowledge as being a collection of information that is "out there," which we collect as we go through life - moving pieces of knowledge from "things I didn't know" to "things I know." But it's important to remember that there are also "things I believe" that may or may not be true, and falsehoods that we wrongly thought of as knowledge (false beliefs). So far this year, a few things that I thought I knew turned out to be false beliefs:
Most people believe that warm water is more effective than cold when washing your hands (one 2013 survey of 510 people found that 69% believed this), and that message is often reinforced in signs we see everywhere like these signs sold by ComplianceSigns.com:
However, this idea turns out to be a complete myth and may even be harmful. As the CDC puts it succinctly: "The temperature of the water does not appear to affect microbe removal; however, warmer water may cause more skin irritation and is more environmentally costly." The same study I mentioned earlier found that teaching people to use a "comfortable" temperature rather than "warm" or "hot" could save one million metric tons (MMt) of CO2eq annually in the US, which is a tiny fraction of overall emissions but still a lot of energy used and emissions created for no good reason.
Here are the sources cited by the CDC:
What about washing laundry in warm water? Again, it's usually not necessary but the answer there is more complex (e.g. some synthetic fabrics like gym clothes don't clean well in cold water). George Dvorsky has an article at io9 that explains some of the details.
When unemployment spikes up and restless young men are frustrated and out of work, violence will inevitably go up, right? So I thought; and I wasn't alone. "The idea that everyone has ingrained into them — that as the economy goes south, crime has to get worse — is wrong," said criminologist David M. Kennedy. “It was never right to begin with.” This is something I learned while reading Steven Pinker's excellent book, The Better Angels of Our Nature: Why Violence Has Declined:
...criminologists have long known that unemployment rates don’t correlate well with rates of violent crime. (They do correlate somewhat with rates of property crime.) Indeed, in the three years after the financial meltdown of 2008, which caused the worst economic downturn since the Great Depression, the American homicide rate fell by another 14 percent...Pinker's book provides a few more examples:
Though unemployment went down in the United States in the 1990s, it went up in Canada, yet violent crime decreased in Canada as well. France and Germany also saw unemployment go up while violence went down, whereas Ireland and the U.K. saw unemployment go down while violence went up.
If you haven't read that book, I highly recommend it - it was one of those books where I wanted to highlight something on every other page. For example, just a page or two later, I learned about another misconception that I held:
In their runaway best seller Freakonomics, University of Chicago economist Steven Levitt and New York Times journalist Stephen J. Dubner put forward a number of fascinating explanations of the world around us using the tools of economics to investigate a wide range of problems seemingly unrelated to economics. One of their most controversial hypotheses was that the decreasing crime in America in the 1990s was due in part to the 1973 legalization of abortion in Roe v. Wade. This was because, as Pinker summarizes, "unwanted children who would ordinarily have grown up to be criminals were not born in the first place, because their begrudging or unfit mothers had had abortions instead."
I remember reading Freakonomics, which I enjoyed immensely, and Levitt's argument about abortions reducing the crime rate made perfect sense to me. The analysis was fairly detailed, and included convincing facts such as their finding that individual states which legalized abortion sooner saw their crime rates fall sooner.
I was quite fascinated then to read Pinker's discussion of this hypothesis in Better Angels, where he discusses it in some detail and provides several reasons why the abortion hypothesis is likely wrong. First, it relies on "the assumptions that legal abortion causes fewer unwanted children, that unwanted children are more likely to become criminals, and that the first abortion-culled generation was the one spearheading the 1990s crime decline." According to Pinker, at least two of these assumptions have either not been proven or have been proven wrong:
To begin with, the freakonomics theory assumes that women were just as likely to have conceived unwanted children before and after 1973, and that the only difference was whether the children were born. But once abortion was legalized [in] 1973 the proportion of children born to women in the most vulnerable categories – poor, single, teenage, and African American – did not decrease, as the freakonomics theory would predict. It increased, and by a lot.
...if easy abortion after 1973 sculpted a more crime-averse generation, the crime decline should have begun with the youngest group and then crept up the age brackets as they got older. The sixteen-year-olds of 1993, for example (who were born in 1977, when abortions were in full swing), should have committed fewer crimes than the sixteen-year-olds of 1983 (who were born in 1967, when abortion was illegal). By similar logic, the twenty-two-year-olds of 1993 should have remained violent, because they were born in pre-Roe 1971. Only in the late 1990s, when the first post-Roe generation reached their twenties, should the twenty-something age bracket have become less violent. In fact, the opposite happened. When the first post-Roe generation came of age in the late 1980s and early 1990s, they did not tug the homicide statistics downward; they indulged in an unprecedented spree of mayhem. The crime decline began when the older cohorts, born well before Roe, laid down their guns and knives, and from them the lower homicide rates trickled down the age scale.
Finally, Pinker tries to go one step further:
What about differences among individual women within a crime-prone population? Here the freakonomics theory would seem to get things backwards. Among women who are accidentally pregnant and unprepared to raise a child, the ones who terminate their pregnancies are likely to be forward-thinking, realistic, and disciplined, whereas the ones who carry the child to term are more likely to be fatalistic, disorganized, or [immature]... Several studies have borne this out. Young pregnant women who opt for abortions get better grades, are less likely to be on welfare, and are more likely to finish school than their counterparts who have miscarriages or carry their pregnancies to term. The availability of abortion thus may have led to a generation that is more prone to crime because it weeded out just the children who, whether through genes or environment, were most likely to exercise maturity and self-control.
We should take this last idea ("abortion...may have led to a generation that is more prone to crime") with a grain of salt, as it seems to be pure speculation; and the overall argument of Pinker's may not be the last word on this subject. But I think it's safe to say that we do not really know with any certainty if or how abortion affected the crime rate.
If you enjoy this sort of thing, make sure you check out Wikipedia's awesome list of common misconceptions - though for the record, I don't think any of these three are on that list.