## Figure 142

The two-string problem in which it is not possible to reach one string while holding the other. other, it was too far away for them to reach (see Figure 14.2). Subjects produced several different types of solutions to this problem but the most "insightful" and infrequently produced one was the pendulum solution. This involved taking the pliers, tying them to one of the strings and swinging it. So, while holding one string, it was possible to catch the other on its up-swing and tie the two together. Maier demonstrated a striking example of "problem restructuring" by first allowing subjects to get to a point where they were stuck and then (apparently accidentally) brushing off the string to set it swinging. Soon after this was done subjects tended to produce the pendulum solution, even though few reported noticing the experimenter brush against the string. According to Maier, this subtle hint resulted in a reorganisation or restructuring of the problem so that the solution emerged (recently, Knoblich & Wartenberg, 1998, have made similar observations under more controlled conditions).

Functional fixedness: The candle problem and nine-dot problem

At around the same time, another young researcher was also expanding Gestalt theory. During his twenties, Karl Duncker (1926, 1945) performed experiments on "functional fixedness" or "functional fixity", that continue to be replicated in various guises to this day (see Kubose, Hummel, & Holyoak, in press). He demonstrated this phenomenon in an experiment where subjects were given a candle, a box of tacks, and several other objects, and asked to attach the candle to a wall by a table, so that it did not drip onto the table below (see Figure 14.3). Duncker found that subjects tried to tack the candle directly to the wall or glue it to the wall by melting it, but few thought of using the inside of the tack-box as a candle holder and tacking it to the wall. In Duncker's terms, subjects were "fixated" on the box's normal function of holding tacks and could not reconceptualise it in a manner that allowed them to solve the problem. Subjects' problem-solving success was hampered by reproductive behaviour (see Weisberg & Suls, 1973, for an information-processing account of the candle problem). Subjects' failure to produce the pendulum solution in the two-string problem can also be seen as a case of functional fixedness because subjects are unable to reconceive of the pliers as a pendulum weight (see Adamson & Taylor, 1954; Keane, 1985a, 1989; Ohlsson, 1992).

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