A small category, only a handful of realistic answers, and a split that depends on a single word in the question.
Name a shape.
The first one that comes to mind. Type it.
Circle or triangle, and performers who use this force pay attention to which verb they used.
Asked to name a shape, people tend toward circle — it is the simplest and the first one learned. Asked to draw one inside another, the classic mentalism setup, triangle becomes far more common, because a circle inside a circle looks like a mistake rather than a choice.
Out of 0 answers so far. Free to quote, with the sample size beside it so a reader can judge the weight.
Once this passes 1000 answers, the measurement replaces the claim above as the headline of this page, and the claim becomes the thing being checked rather than the thing being reported. Saying that in advance is the point: a threshold decided afterwards is not a threshold.
Most of these questions have hundreds of possible answers. This one has perhaps eight that a person would produce without effort: circle, square, triangle, rectangle, star, heart, diamond, oval. That should make it the most predictable force here, and it nearly is.
An even spread across eight would give 12.5% each. Anything the leading answer takes above that is the prototype effect, and with a category this small it should be easy to see.
"Name a shape" and "draw a shape" are different questions, and the classic mentalism version asks for a drawing — usually a simple shape drawn inside another simple shape.
That framing pushes people toward variety. Having drawn a circle they will not draw a circle inside it, because it reads as a failure to follow the instruction rather than a choice. So the second shape is nearly always different from the first, and triangle, which is distinct from everything, does well.
This page asks people to name one, so the numbers should lean toward circle. That difference is worth watching if you perform the drawing version, because a prediction built on one question will misfire on the other.
Predict two shapes rather than one, which is what the drawing version has always done: a triangle inside a circle covers the common answer and the common pairing at once. It looks like a bolder prediction than it is.
Circle is the expected answer when people are asked to name one. It is the simplest shape and the first one learned.
Triangle becomes more common when people are asked to draw a shape inside another, because repeating the first shape feels like a mistake.
Because drawing one shape inside another pushes people toward variety. Having drawn a circle they avoid a second circle, so the inner shape is nearly always different.
A prediction built on "name a shape" will misfire on "draw a shape", and the reverse.
About eight: circle, square, triangle, rectangle, star, heart, diamond and oval. An even spread would give each 12.5%.
Anything the leading answer takes above that is the prototype effect, and a small category makes it easy to see.
This is one of eleven questions in the full study, which asks for a card, a number, a vegetable, a fruit, an animal and more, and shows the running counts for all of them. The rest of the site is self-working card tricks you can play in the browser.