A theory proposed by the US psychologist Harry Helson (1898–1977) in an article in the American Journal of Psychology in 1947, according to which the adaptation level is determined for a class of stimuli by members of the class already sampled or attended to, by stimuli having a background or contextual influence, and by recollections of past judgements of similar stimuli, the adaptation level being the logarithm of the mean of the relevant stimuli, weighted according to their effectiveness in terms of nearness, recency, salience, and so on. According to the theory, subjective judgements are necessarily relative to the prevailing norm or adaptation level, so that a 4 ounce (113 gram) pen is heavy, but a baseball bat must weigh over 40 ounces (1.13 kilograms) to be judged heavy. The phenomenon is a type of context effect. Also called AL theory. See also hedonic treadmill, prospect theory. Compare assimilation-contrast theory.