Imagine you are a caveman, and encounter two fruits: one that you’ve seen before, and one that you haven’t. Which are you more likely to eat? The cavemen who picked the bush they hadn’t seen before tended not to survive as long (for obvious reasons). For this reason, we have more positive feelings about people and things we have seen before than those we haven’t seen before. This effect exists even when we have seen something neutrally; positive exposure is not necessary. Well it might be a case of the mere-exposure effect.
The earliest scientific recordings of the effect came from the work of Fechner (1876), and Titchener (1910), who referred to the “glow of warmth” felt in the presence of something that is familiar. The effect was later thoroughly investigated by Robert Zajonc (1968). In his experiments he tested how subjects responded to made-up words and Chinese characters and found those exposed to these words the most responded the most favourably to them. People tend to develop a liking for something after repeated exposure.
The effect was even found among animals. Two frequencies were played to different groups of chicks while they were still unhatched, and after hatching each set of chicks consistently chose the tone played to it in the pre-natal stage (Zajonc, 2001). The effect typically reaches its peak after 10-20 exposures, and declining thereafter (Bornstein, 1989).