There are a bunch of behaviors that have been shown in one or more studies to make people stupider.
Here's a list of things that decrease your IQ or cause neurological decline. Can you add to the list?
Watching reality TV
An Austrian study showed participants a reality-like show and asked them to take a knowledge test immediately afterward. Those participants fared worse than those who had not seen the reality show beforehand.
A UCLA study showed that steady sugar consumption for as little as six weeks "slows the brain, hampering memory and learning." Americans consume 35 pounds of high-fructose corn syrup each year.
Research conducted at Stanford University in 2009 shows that multitaskers "who are regularly bombarded with several streams of electronic information do not pay attention, control their memory or switch from one job to another as well as those who prefer to complete one task at a time."
A series of three experiments conducted by Cardiff University in Wales determined that chewing gum "impairs short-term memory for both item order and item identity."
A 2010 Kent State University study tested more than 100 obese individuals before and after they had bariatric surgery. Men's Health reports that "before the surgery, most subjects showed below-average memory skills. But 12 weeks after surgery...their memory test scores had improved to within the average range for all adults."
Researchers at Cal Berkeley changed the sleep schedule for hamsters every three days for a month and the hamsters produced 50 percent fewer neurons than they did on a normal sleep schedule.
A recent study concluded that "children in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas." Fluoride is found in most drinking water in the United States.
In businesses around the world, it's fairly common to toss ideas around at meeting to help stimulate creative and productive activity. But a Virginia Tech study revealed that "group settings can diminish expressions of intelligence, especially among women." Social feedback "had a significant effect" on the subjects' problem-solving abilities.
Being spanked as a child
A wide-ranging study by the University of Manitoba found that more than five percent of all mental disorder is caused by being spanked or other forms of childhood physical abuse.
If you believe the U.S. Army, PowerPoint presentations are making us stupid. Commanders in the Army told the New York Times in 2010 that the Microsoft program "stifles discussion, critical thinking and thoughtful decision-making."
A 2011 study by the journal Pediatrics showed that children who watched fast-paced cartoons, like SpongeBob, performed poorer at a mental test than those who watched an educational show or those who drew.
In addition to the numerous other harmful effects secondhand smoke causes, children who are exposed to enough of it could end up with lower IQs and lower achievement in school and on test scores, according to Central Michigan University.
The Yale Stress Center concluded this year that stressful situations "can reduce the number of connections between neurons in the brain and impair the ability of managing tense events in the future."
Smoking weed consistently, from adolescence to adulthood, causes "neuropsychological decline." The Duke study found that even stopping the habit for a long period of time "did not fully restore neuropsychological functioning."