I've read that in the 19th century, there were many people who said that iron ships couldn't possibly float. If you take a few seconds to do the math, you can quickly verify that iron ships can float. That seems like a good slam-dunk to me. Is there a modern equivalent?

Some similar, not-quite-as-obvious former popular opinions:

• Humans can never fly.
• Humans can never reach the moon.

(Interestingly, the much older "sailboats can never sail upwind" seems more plausible to me than any of these.)

Contemporary unpopular slam-dunk-yes views:

• Computers will someday attain human-level performance on any task you can name.
• Technology can enable humans to live to the age of 200.
• Evolution. (Still unpopular worldwide.)

There are culture-specific slam-dunks. I noticed, while traveling in China, particularly during an episode when the US bombed a Chinese embassy and when discussing the Tienanmen Square massacre, that most of the Chinese people who spoke openly with me (just a few) simultaneously believe their government is corrupt and untrustworthy, yet believed everything it said about those incidents. Numerous Russians I've spoken to have a blindness reconciling their views on Stalin with their views on Putin (the same attributes that made Stalin bad make Putin good). Maybe a foreigner should help us identify our slam-dunks.

There are some slam-dunks that are popular on the low end of g, and on the high end of g, but not in the middle range of g, e.g.

• Men and women have different distributions of preferences and cognitive aptitudes.

Using these in your survey could contaminate the results.