The tendency for repeated exposure to a stimulus to be sufficient to enhance an observer's liking for it or attitude towards it. The effect was first referred to by the German philosopher, physician, psychologist, and mystic Gustav Theodor Fechner (1801–87) in Vorschule der Aesthetik (1876, pp. 240–3), and the US psychologist William James (1842–1910) independently rediscovered it and discussed it in Principles of Psychology (1890, volume 2, p. 672), but the first quantitative investigation of it was carried out by the US-based Polish psychologist Robert B(oleslaw) Zajonc (1923–2008) and published in the journal Psychological Monographs in 1968. Zajonc examined antonym pairs and found that the positively toned words were more frequent in the language according to word counts than negatively toned words: happiness occurs more than 15 times as frequently in written English as unhappiness, beauty 41 times as frequently as ugliness, love almost 10 times as frequently as hate, and so on, and similar relationships between frequency and favourability have been found in French, German, Spanish, Russian, Urdu, and other languages. Zajonc also reported experimental evidence using pseudo-Turkish nonsense words such as iktitaf and diagrams resembling Chinese logographs. Participants were exposed to different stimuli different numbers of times, and they then guessed the degree of favourability of the meanings of these items. The results showed almost linear relationships between (log transformed) exposure frequency and rated favourability. See also Pollyanna effect, preference-feedback hypothesis.