It's not that because people's inflammation is less they are more likely to do things they wouldn't before because of pain, but as a side effect they engage in riskier behavior. The test they used was to have participants inflate a balloon. The bigger the balloon the more imaginary money they'd win. When given 1g of tylenol people were more likely to pop that balloon compared to people who didn't take the tylenol.

The cognitive decline was determined from another test. I'll just c&p that one.

Next, the participants performed a test that involved hitting a certain button when an "F" appeared on a computer screen, and not hitting the button when an "E" appeared. This is called a No/NoGo test, and it basically gauges how fast and accurate a person’s decisions are.

When all was said and done, the group that took the Tylenol ended up hitting the button when "E" appeared and also missed the "F" pop-ups more frequently than those who took the placebo. According to the team, this indicates that Tylenol could make it harder for a person to make accurate, quick decisions.