People who are smart, or think they are smart, often criticize others for their inability to "connect the dots," or to see a pattern that makes sense of otherwise unconnected information.
But both smart people and not-so-smart people have problems with overgeneralization. That is, once they have figured out or learned a general rule, they sometimes fail to see when there are exceptions to the rule.
In other words, they connect dots that, in reality, are not connected. We all do it. It is one of the difficulties of life.
Partly this is driven by necessity, partly by laziness.
We all have limited time. For any given task we must limit the time we can commit, otherwise the many other tasks in our life will not get done. This is true in decision making and in intellectual pursuits as well as daily tasks.
Limits on decision making time are often externally imposed. Most American citizens don't devote very much time to politics, for instance. A fair proportion of citizens vote in elections, and there is a deadline for each election. We are only willing to devote so much time to learning about the candidates and choosing between them. We might listen to ads, if not willingly, and some voters listen to debates. But how many voters go over a candidate's voting record? And even if a citizen had nothing else to do, to actually read all the words of all the legislation that elected officials vote on is impossible. Even the politicians don't do it: they rely on their staffs and on the work of the committees that write the legislation.
So we generalize. We let simple criteria guide us. In most general elections most voters simply vote either Democratic Party or Republican Party. Primaries are more difficult, because the choices are within a party. That is one reason so few people vote in primaries: they don't know who to vote for. Some people vote based on a key issue like Social Security or pro-life/pro-choice, or based on perceptions of personality, or even just handsomeness.
Generalizations can be untrue, but the more difficult cases are when they are mostly true, but have important exceptions. Since the beginning of the science of astronomy, objects in the sky were classified into the sun, moon, planets, and stars. But when a sufficiently powerful telescope was developed, it turned out some of the stars were actually galaxies. So to every animal that swims is not a fish: some are marine mammals.
One of my favorite areas to watch people overgeneralize is in food, diet, and health. The best example right now is glutenphobia. Gluten, the protein component of wheat, can cause reactions in individuals whose immune systems are out of balance. But this is rare. Yet by constantly complaining, these gluten-intolerant individuals got food companies to note which foods are gluten-free. Other people (most people thrive on gluten) started seeing the words "gluten free" on labels and decided that gluten must be a poison. Quack doctors, pseudoscientists and "health food" corporations realized they could make a lot of quick bucks by promoting this fear.
Fear and hope are big drivers towards overgeneralization in ordinary life. Barked at by a dog? Beware of all dogs. Win a jackpot at a casino? Lose all your money trying to hit another jackpot.
Fear can save your life, of course. Not to long ago, in a state of nature, when there were still lions and tigers and wolves and bears to worry about, fear was a friend. Fear kept people alive. Hoping to kill a grizzly bear alone with a flint knife was a bad use of hope. Somewhat in the same way that people now get immunological diseases because their immune systems are not exposed to enough bacteria and viruses, now our fears tend towards the irrational. Our fear system overgeneralizes.
Almost everyone has life experiences that show us that some particular overgeneralization is wrong. As a child I was taught Jews were bad people who had killed Jesus. Anti-jewish remarks were a commonplace where I went to school (Roman Catholic Schools) through 8th grade. In 9th grade, at a different school, I made the usual anti-semitic remarks. Imagine my embarrassment when I learned that many of the students in my classes were Jewish, and that they tended to be the kids I wanted to be friends with. Fortunately they were gracious and came to accept me, once I stopped talking like a jackass.
By now police in America should know that being a black and a teenage male does not mean you are a criminal. Policing can be a difficult job, but that is no excuse for making judgments about people based on appearances. A poorly-dressed person may be poorly-dressed precisely because he (or she) is not as greedy and unscrupulous and the people in nice suits.