In the 1960s, a series of laboratory experiments by Robert Zajonc demonstrated that simply exposing subjects to a familiar stimulus led them to rate it more positively than other, similar stimuli which had not been presented.
In the beginning of his research, Zajonc looked at language and the frequency of words used. He found that overall positive words received more usage than their negative counterparts. In later years, he moved on to show similar results for a variety of stimuli such as polygons, drawings, photographs of expressions, nonsense words, and idiographs, as well as when being judged by a variety of procedures such as liking, pleasantness, and forced-choice measures.
Zajonc sought to provide evidence for the affective-primacy hypothesis, namely, that affective judgments are made without prior cognitive processes.
In 1980, Zajonc proposed the affective primacy hypothesis, which hypothesizes that affective reactions (i.e. liking) can be “elicited with minimal stimulus input”. Through mere-exposure experiments, Zajonc sought to provide evidence for the affective-primacy hypothesis, namely, that affective judgments are made without prior cognitive processes. Zajonc tested this hypothesis by presenting repeated stimuli to participants at suboptimal thresholds such that they did not show conscious awareness or recognition of the repeated stimuli (when asked whether they had seen the image, responses at chance level), but continued to show affective bias towards the repeatedly exposed stimuli. Zajonc compared results from primes exposed longer which allowed for conscious awareness to stimuli shown briefly such that participants did not show conscious awareness. He found that the primes shown more briefly and not recognized prompted faster responses for liking as compared to primes shown at conscious levels.
One experiment that was conducted to test the mere-exposure effect used fertile chicken eggs for the test subjects. Tones of two different frequencies were played to different groups of chicks while they were still unhatched.
Once hatched, each tone was played to both groups of chicks. Each set of chicks consistently chose the tone prenatally played to it. Zajonc tested the mere-exposure effect by exposing Chinese characters for shorts amounts of time to two groups of individuals. The individuals were then told that these symbols represented adjectives and were asked to rate whether the symbols held positive or negative connotations. The symbols that had been previously seen by the test subjects were consistently rated more positively than those unseen. In a similar experiment, the individuals were not aksed to rate the connotations of the symbols, but to describe their mood after the experiment. Members of the group with repeated exposure to certain characters reported being in better moods and felt more positive than those who did not receive repeated exposure.