The social penetration theory (SPT) proposes that, as relationships develop, interpersonal communication moves from relatively shallow, non-intimate levels to deeper, more intimate ones. The theory was formulated by psychologists Irwin Altman, of The University of Utah and Dalmas Taylor, of The University of Delaware in 1973 to understand relationship development between individuals. Altman and Taylor note that relationships "involve different levels of intimacy of exchange or degree of social penetration". SPT is known as an objective theory as opposed to an interpretive theory, meaning that it is based on data drawn from experiments and not from conclusions based on individuals' specific experiences.