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The genetics of social behavior presents special difficulties because the phenotype is the product of an interaction between two or more individuals. Social interactions are of two kinds: (1) cooperative, in which the probabilities of transmission of the genes of all participants are similarly affected by the outcome, and (2) agonistic, in which the probabilities for the participants are affected in opposite directions. The latter are of particular interest for evolutionary theory. Three major types of designs for measuring social behavior in genetic experiments are available: (1) homogeneous sets, (2) standard tester, and (3) tester panel representing a reference population. The advantages and limitations of each method are discussed. Important areas for future development include the relationship of genetic and experiential factors in early life to social status as an adult and the extension of the genetic analysis of social behavior to natural populations.

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Psychomotor development from birth to 18 days of, age was observed in 3 Lines of mice selected for high (H), medium (M), and low (L) brain weight. Litters were cut to 8 at birth and half of the individuals randomly assigned to a practiced group tested daily; the remainder were assigned to a nonpracticed group and were tested only on selected tests on the day at which their littermates met a predesignated criterion. Significant differences between the lines were found in open field activity, balancing on a rotorod, age of eye-opening, and edge avoidance. The H-line was generally more advanced than either M or L. On some measures, righting for example, no strain variation was found. The effects of practice were usually significant, but did not abolish strain differences.Most research on the behavioral effects of genetic variation in the nervous system has been concerned with effects of mutant genes producing major disruption of developmental patterns. Our results indicate that genes affecting the rate and amount of normal brain growth also influence behavioral development. Genes of this type may be responsible for some of the individuality of behavioral phenotype found in natural populations.Previous research on the physical development of mice selected for differences in brain weight (Roderick and Wimer's SEL-16) has shown that brain size of the high line surpasses controls at birth, whereas brain size of the low line at birth is similar to controls but growth slackens more abruptly early in the 2nd week (Fuller & Geils, 1972). On behavioral tests given over the first 3 weeks of life the high line generally surpasses controls at a comparable age. The low line also tends to develop behaviorally more rapidly than controls, but the effect is inconsistent (Fuller & Geils, 1973). These authors pointed out that correlations between behavioral and physical characteristics may be due to common environmental dependencies, or to 1 of 3 kinds of genetic communalities. Only 1 of these, common dependence of behavioral and physical variation upon the same segregating alleles, is of interest to students of brain-behavior relationships. One way of strengthening the case for such pleiotropic effects is to demonstrate similar brain-behavior correlations in independently derived lines.