The contact hypothesis was proposed by researchers in the mid-20th century and they are interested in understanding how to reduce conflict and prejudice. For example, studies in the 1940s and 1950s found that exposure to other members of the group was associated with a lower degree of bias. In a study conducted in 1951, the researchers studied the relationship between life and prejudice in isolated or separated housing units and found that in New York (where housing is isolated), white research participants were more biased than in New York. WACKER (housing is white) has low white participants. Still isolated). Harvard psychologist Gordon Allport was one of the key early theorists to study the contact hypothesis. In 1954 he published an influential book, The Essence of Prejudice. In his book, Allport reviews previous research on inter-group contact and prejudice. He found that in some cases, exposure reduces prejudice, but this is not a panacea, and in some cases, inter-group contacts also exacerbate prejudice and conflict. In order to solve this problem, Allport tried to find out when the contact was successful and reduced the prejudice. He proposed four conditions, which were later studied by researchers.