Functional fixedness vs mental set: the difference, and the experiments that named them

Functional fixedness is the tendency to see an object only in terms of its usual use. Thus you do not notice a new use that a problem needs. Mental set is the tendency to use a method that worked before, even when a new problem needs a simpler or different method. The difference is what stays fixed: the job of an object, or the method. Karl Duncker described functional fixedness in 1935, and Abraham Luchins measured mental set with water jar problems in 1942.

Most summaries give one example for each term and stop there. This article gives you one question that sorts any example. It also gives you the original experiments with their numbers, and what helps people find the better solution.

• Functional fixedness: you see an object only in its usual function, so you miss the new function that the problem needs.
• Mental set: you repeat a method that worked on earlier problems, so you miss a simpler or better method.
• The sorting question is: is the person stuck on what an object can do, or on how to do the task?
• Many textbooks treat functional fixedness as a type of mental set. When the block is about the use of an object, the more specific answer is functional fixedness.
• Neither effect is total. In Duncker's box problem, 3 of 7 people still found the solution after the boxes held other things.

The difference in one table

The two terms describe the same kind of obstacle: earlier experience blocks a better solution. The table shows where they differ.

The one question that sorts any example

Exam questions describe a person who is stuck. To name the obstacle, ask what stays fixed. Use these steps.

1. 1.Find the point where the person is stuck.
2. 2.Find what the person does not change.
3. 3.If the person does not see a new use for an object, the answer is functional fixedness.
4. 4.If the person repeats a method that worked on earlier problems, the answer is mental set.
5. 5.If both seem to apply, choose functional fixedness when the block is about an object. It is the more specific term.

All six situations in the next table come from the original studies.

Functional fixedness: the object keeps its old job

Karl Duncker's monograph On Problem-Solving is the English translation of a German book that he published in 1935. In it, Duncker described a "fixedness" that comes from use. His example is simple: "a stick that has just been used as a ruler is less likely to appear as a tool for other purposes". He called this effect functional fixedness. He also wrote that his results for objects were "valid, in principle, for thought-material in general".

Duncker's box problem

The task was to put three small candles side by side on a door, at the height of the eyes. On a table, among many other objects, there were some tacks and three small pasteboard boxes. Each box was about the size of a matchbox. The solution is to tack each box to the door and use it as a platform for a candle.

Each problem had two settings. In one setting, the boxes were empty. In the other setting, the boxes held the material for the task. One box held candles, one held tacks and one held matches. In that setting, the boxes first had the function of a container. Duncker used four more problems of the same type, with a gimlet, pliers, a weight and a paperclip.

Duncker concluded that "the object which is not fixed is almost twice as easily found as the object which is fixed."

One more result surprised him. In a variation, the boxes held buttons, not the candles, tacks and matches. Then only 1 of 7 participants solved the problem. Duncker's explanation was contact. The candles are central to the task, so people usually empty the candle box first. A box of buttons stays at the edge of the task.

Many modern descriptions use one candle, a box of tacks and a book of matches. The OpenStax psychology textbook uses this version, for example. Duncker used three candles and three boxes. The idea is the same, but his numbers come from his own version.

The repetition with 57 students

Robert Adamson wrote in 1952 that Duncker did the experiments "with but 14 Ss and under poorly specified experimental conditions". Adamson, at Stanford University, repeated three of the problems with 57 college students. His box problem also used three candles, and each student had a maximum of 20 minutes.

In the control group, the boxes were empty, and 24 of 28 students (86%) solved the box problem. In the other group, the boxes held the candles, matches and tacks, and 12 of 29 students (41%) solved it. The difference was statistically significant. Adamson's larger sample thus confirmed Duncker's main result.

Mental set: the method keeps its old job

The APA Dictionary of Psychology defines mental set as "a temporary readiness to perform certain psychological functions". Its example is "the tendency to apply a previously successful technique in solving a new problem". The dictionary says that the term is "essentially synonymous with the older term Einstellung". Einstellung is one of the German words that English psychology kept, as our list of psychology terms that do not translate cleanly into German shows.

Luchins' water jars

Abraham Luchins published the classic study in 1942. Each problem gives you three empty jars and a target amount of water. You must get the exact amount, on paper. The table shows four of the problems.

Problems 2 to 6 all need the long method, B − A − 2C. You fill jar B, then you fill jar A from it once and jar C twice. Problems 7 and 8 also allow the long method, but a shorter method works.

Luchins first tried the problems in 1936, in a seminar of the psychologist Max Wertheimer. The group had 15 people, and many of them had a Ph.D. or an M.D. degree. All 11 people who got only the normal instructions used the long method on problems 7 and 8. Not one person used the short method. Afterwards, several of them said "How dumb I was" or "How blind I was".

The 4 other people got a secret instruction: after problem 6, write the words "Don't be blind!" on the paper. In 5 of their 8 answers to problems 7 and 8, they used the short method. The warning helped, but it did not remove the effect.

Luchins then tested many school classes and adult groups. In six college classes, the people without the warning used the long method in 70 to 100% of their answers to problems 7 and 8. Two control classes did not get problems 2 to 6. They used the short method in all of their answers to problems 7 and 8. In one variation, 38 school pupils got ten practice problems for the long method. Then almost all of them failed to solve problem 9, which only the short method solves.

Luchins called the habit an Einstellung: "the habituation to the repeatedly used procedure". Thus the method itself became fixed, not an object.

Experts are not immune

One idea is that expertise makes thought rigid, and the opposite idea is that it makes thought flexible. Merim Bilalić, Peter McLeod and Fernand Gobet tested both ideas with chess players. Each problem had two solutions: a familiar mate in five moves, the smothered mate, and a less familiar mate in three moves.

In their 2008 study, the familiar solution caused the Einstellung effect "even in experts". It reduced the experts' problem-solving ability to about the level of players "three standard deviations lower in skill level". But the effect became smaller as skill increased. The authors wrote that "the more expert they were, the less prone they were to the effect".

A second 2008 study recorded eye movements. After the experts found the familiar solution, they said that they were looking for a better one. But their eyes continued to go to the parts of the board that belonged to the first solution. The authors suggest that the same mechanism may contribute to confirmation bias. Our list of ten cognitive biases explains confirmation bias.

Where the two terms overlap

The two terms overlap. The OpenStax textbook says that "functional fixedness is a type of mental set". The textbook gives no reason, but one reason is clear from the studies: both effects come from earlier experience. Duncker's fixedness came from a use earlier in the same task, and Luchins' set came from earlier success with one method.

The two studies also have the same origin. Luchins wrote that his water jar problem was "first investigated in the Berlin Institute of Psychology by Zener and Duncker". Those experiments were preliminary, and they had not been published by 1942. Luchins did his own study under the sponsorship of Max Wertheimer, and Duncker dedicated his 1935 book to Wolfgang Köhler and Max Wertheimer.

For an exam, the overlap means that both answers can be correct for an object example. But functional fixedness, the more specific term, gives the more precise answer.

What helps, and what the evidence cannot tell you

The classic studies also tested ways to remove the block. The results are useful, but they have limits.

• A warning helps a little, as the "Don't be blind!" result above shows.
• A problem that the old method cannot solve helps some people. In Luchins' seminar, the group without the warning gave no short-method answers before problem 9. On the two problems after it, 15% of its answers used the short method.
• A description of an object by its parts helps. Tony McCaffrey trained people in a generic parts technique. You divide an object into parts, and you ask if the description of each part implies a use. If it does, you use a more general description: a candle wick becomes a piece of string. In McCaffrey's study, the trained group "solved on average 67% more problems than a control group did".
• Young children show less functional fixedness. Tim German and Margaret Anne Defeyter tested this in 2000. Children of 6 and 7 were slower to use a box as a support after they saw its container function. The 5-year-olds were "immune to this effect".
• A habit is not a fault. Luchins wrote that "mechanized responses have a place in one's behavior". The danger begins when "instead of the individual mastering the habit, the habit masters the individual".

Duncker's sample was small, as Adamson noted. Adamson's repetition and the chess studies support the main results. But these numbers do not tell you how strong either effect is in your own daily life.

How to keep the two terms apart

Two memory aids come from the words themselves.

• Functional fixedness: the function is fixed. Ask what the object could do.
• Mental set: the method is set. Ask how else you could do the task.

Then test yourself. Do not read the definitions again. Take a new example, cover the answer, and apply the sorting question. Luchins gave teachers similar advice: mix in problems that the method you just learned cannot solve. Our guide to memorising psychology terminology explains why tests help you remember more than a second reading does.

Psychology World is a dictionary app with more than 18,000 psychology terms, and it supports this type of practice. Search the dictionary for functional fixedness, mental set and Einstellung, and save the terms to one bookmark list. Then study the list as flashcards, as multiple-choice questions, or type each answer. The multiple-choice mode needs a list with a minimum of four terms. The home page shows the four steps of the learning loop and what each study mode does.