Human long-term memory is characterized by a nearly limitless storage capacity. At any time, however, much of the information that exists in long-term memories (names, numbers, facts, procedures, events, and so forth) is not recallable. Why do people forget information that was once recallable? Because access to information in memory is subject to interference from competing information in memory. Before characterizing such interference processes in more detail, it is necessary to introduce some terminology.

The first concept is transfer. After some early point in life, people rarely, if ever, learn anything that is entirely new. Rather, people bring to any "new" learning of knowledge or skills an accumulation of related knowledge, skills, and habits from the past. Such prior learning influences the qualitative and quantitative character of the new learning process. Such transfer effects may be positive or negative, depending on whether prior experiences...