It's known as the Flynn Effect, for moral philosopher James Flynn.
"The cars that people drove in 1900 have altered because the roads are better and because of technology," Flynn said in a TED Talk last year. "And our minds have altered, too. We've gone from people who confronted a concrete world and analyzed that world primarily in terms of how much it would benefit them to people who confront a very complex world."
For instance, education has changed. We've learned to classify the world, to compare groups like animals or modes of transportation, Flynn said. We've also been taught to accept hypothetical situations (you remember algebra, right?). Our ancestors dealt only with what was right in front of them.
Our jobs have also changed. In the early 1900s, only 3 percent of Americans had professions that were "cognitively demanding," Flynn said. Today, 35 percent of us do. As such we're used to solving complex, hypothetical problems, like the ones on an IQ test.
Health factors may have had an influence as well. Studies have shown that early childhood immunization rates are a big predictor of a nation's average IQ score. So decreasing infectious diseases worldwide may have attributed to the overall increase in subsequent generations' IQ scores.
"From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks," the authors of one study wrote.
Not a genius? Don't panic
You probably remember the dreaded SAT or ACT test you took in high school. That's a type of IQ test. But Nisbett believes that a student's grade-point average is a better predictor of their success than their test scores.
"GPA is raw smarts times how hard you work times self-control times a lot of other things. That's true for success in life," he said. "I see graduate students with extremely high IQs who can't achieve much because they're lacking in curiosity. ...They're lacking the ability to get along with people."
Having a high IQ is not a guarantee of success, Van Gemert agrees, just as having a lower IQ is not a guarantee of failure. Good habits, perseverance and a strong work ethic are just as important as intelligence.
"If you don't develop those other qualities, you can waste a smart IQ," she said.
Van Gemert recommends that parents view their homes as a petri dish, one where they're trying to grow their children. That means lots of time spent together, interacting, and lots of books, building blocks and board games.
"The most important thing we can do for kids is to play with them," she said.