approaches assume that the commonalities and differences are independent of each other. However, commonalities and differences are not psychologically independent. In fact, determining the differences between a pair requires finding the commonalities. Consider the comparison between a car and a motorcycle. Both have wheels. That is a commonality. However, cars have four wheels, while motorcycles have two wheels. That is a difference. Because this difference required first finding a commonality between the pair, it is called an alignable difference. Alignable differences contrast with nonalignable differences which are aspects of one concept that have no correspondence in the other. For example, cars have seatbelts and motorcycles do not. Research suggests that alignable differences have a larger impact on people's judgments of similarity than do nonalignable differences. Thus, the relationship between the commonalities of a pair and the differences is important for understanding people's assessments of similarity. Structural approaches to similarity emerged from research on analogy. Transformational approaches Transformational accounts of similarity were developed to evaluate similarity independently of the type of mental representation. On this view, any mental representation can be transformed into another mental representation through some series of steps. For any representation system and set of transformations, it is possible to define the shortest set of steps (i.e., the shortest program) that will transform one representation into another. The shorter this minimal program, the more similarity the pair of concepts. found some evidence against this view, showing that the number of steps to transform the colors and shapes of geometric objects does not predict people's similarity judgments for those objects. Social psychological approaches In social psychology large amounts of empirical evidence indicate that similarity breeds liking; this is known as the similarity effect. Similarity refers to personality, attitudes, values, interests, and attraction shared between to individuals. Similarity is closely related to Bryne's social psychology model of interpersonal attraction (1961) which is determined by four variables: propinquity (how our environment and situation play a role in determining how often and to what degree we come in contact), need for affiliation, overt stimulus characteristics (refers to the observable attributes of an individual that serve to elicit positive or negative responses from others), and similarity. In short we determine our attraction or liking of another by positive and negative reinforcements and our emotions concerning the matter. When Individual A receives positive reinforcement from individual B, A's attraction toward B, increases the reverse