In theory, this saying cannot be true. This is because the average of a set of numbers is calculated by adding up all the numbers and then dividing by the total number of values.
Let's assume we have 100 people.
If 90% of people say they are smarter than the average, that means 90 people think they are smarter than the average.
However, since the average is calculated using all the values, it is mathematically impossible for 90% of people to be above average. This is because the average will always be the middle value when all values are ordered from least to greatest.
Therefore, the saying is a paradox and not logically possible.
\textbf{Answer:} The saying "90% of people say they are smarter than the average" is not true in theory.