Identical (monozygotic) twins have attracted special attention for the study of behavior genetics. Some of the assumptions and results of these studies are reviewed with special attention given to the natural experiment of identical twins adopted by different families. However, the correlation for any behavior between the adopted twins of a monozygotic pair is affected by their common prenatal environment as well as by the pervasive similarity of the two adoptive environments. The genetic contribution to complex social phenomena, but also to physical characteristics such as height, is usually overestimated. Both for adopted and unadopted twins differential effects of cultural, family, and prenatal environments, and the correlated experiences of twins and their physical appearance affect their development. Taxonomies and samples of environments need be examined in order to be able to estimate the genetic contributions to behavioral traits.

The well documented cognitive revolution was to a large extent an evolving return to attitudes and trends that were present prior to the advent of behaviorism and that were alive and well outside of the United States, where behaviorism had not developed any coherent support. The behaviorism of the 1920 to 1950 period was replaced because it was unable to address central issues in human psychology, a failure that was inherent in part in J. B. Watson's founding manifesto with its insistence on the seamless continuity of human and nonhuman animal behavior. The "revolution" was often slow and piece meal, as illustrated by four conferences held between 1955 and 1966 in the field of memory. With the realization that different approaches and concepts were needed to address a psychology of the human, developments in German, British and Francophone psychology provided some of the fuel of the revolution.

Recognition memory and recall/recollection are the major divisions of the psychology of human memory. Theories of recognition have shifted from a ‘‘strength’’ approach to a dual-process view, which distinguishes between knowing that one has experienced an object before and knowing what it was. In this article, I discuss the history of this approach and the two processes of familiarity and recollection and locate their origin in pattern matching and organization. I evaluate various theories in terms of their basic requirements and their defining research and propose the extension of the original two process theory to domains such as pictorial recognition. Finally, I present the main phenomena that a dual-process theory of recognition must account for and discuss future needs and directions of research and development.

Experimental psychology in the early 20th century was targeted by several authors who described a crisis— often expressed as a lack of theoretical and experimental progress.

In the 21st century, the crisis of competing theories has been largely overcome but several current emphases hinder the development of a mature experimental science.

Central among these are an ethnocentrism that focuses on Western standards and populations, neuroscientism which often treats neurological evidence independently of mental and behavioral events, and the tendency for demonstration experiments to replace coordinated theoretical approaches.

Recent as well as historical critiques of association theory, which is based on automatic linking mechanisms, lead to a consideration of an alternative: organization theory. The latter theory postulates that human memory is organized, instead, in a nested, hierarchical fashion that structures storage and retrieval. Organization theory also postulates that there is a limit of four expandable units in organized memory and that the same limit is found in recall and production units.

The consciousness continuum is seen as extending from simple sensory experiences to complex subjective constructions resulting in the apparent exercise of conscious will. The phenomena between these two extremes include spontaneously occurring mental contents, unintended perceptual experiences, memory retrievals, and problem solving including feedback of conscious contents. Two factors describe this continuum: The presence or absence of intention (psychologically defined) and the complexity of the cognitive construction involved. Among other benefits, such an analysis is intended to provide an alternative to metaphysical and vague concepts such as qualia, free will, and intentionality.