The phenomenon of monogamy is complex both because different characteristics associated with monogamy are displayed by different species and because different authors tend to focus on different characteristics, in part because of the species they study. The essence of monogamy appears to lie in three dimensions--exclusivity of mating, shared parental care, and association. However, many species that can be treated as "monogamous" may fail to show one or more of these characteristics. No one characteristic can be taken as definitive of monogamy. As a rule of thumb, we might consider a system monogamous if two of the three dimensions of monogamy are present. Monogamy, then, is not a unitary construct, but a general term that is useful in delineating a range of phenomena. A fascinating set of questions concerns the determinants of exclusivity of mating, shared parental care, and different kinds of association and when each is and is not shown. This search can be hindered by overreliance on "monogamy" conceived as a unitary characteristic. I find myself following in the footsteps of Frank Beach's Nebraska Symposium paper of 30 years ago in arguing against a unitary concept of monogamy, just as he argued against a unitary concept of "sex drive." Mating systems, such as monogamy, are the product of the behavioral patterns displayed by individual organisms. Our work on voles, like that of Mason and his associates on primates, reveals important differences in the motivational systems of monogamous and nonmonogamous species. Animals show both plasticity within species, as individuals encounter different conditions, and remarkably stable species differences, as laboratory-reared individuals vary reliably. The delineation of these personality profiles of species displaying different proclivities toward mating systems illustrates the utility of the comparative method and can help unravel the underlying causes of a variety of psychological differences, from those affecting mate choice to sex differences in spatial learning. They are thus fundamental to a comprehensive understanding of any species. Although the goal of comparative psychology lies in generating principles of generality concerning behavior rather than in direct extrapolation to humans, results can provide worthwhile hypotheses regarding the evolution of human behavior. Although monogamy and mate choice in humans may be regulated by underlying processes different from those in other species, there are many functional similarities, and both are ultimately the products of natural selection.(ABSTRACT TRUNCATED AT 400 WORDS)