An analysis of the performance of Ss at a number of age levels, from 3 yr. to 20 yr., in a problem-solving task. Dependent variables include terminal level of correct response, rate of rise to terminal level of response, patterns of response, and performance as a function of a preceding correct or incorrect response. A U-shaped relationship between age and terminal level of correct response is reported, while age and simple patterns of response bear an inverted U relationship to one another. Younger Ss show a more rapid rise to terminal level of response than do older Ss. In addition, differential effects of reinforcement and nonreinforcement are noted among the age groups, and differential changes occur in these effects as the task progresses. Data from a number of other studies are discussed in relation to these findings, and an attempt is made to consolidate various sources of evidence to provide support for the mechanisms which are assumed to underlie certain of the developmental changes reported. Differential growth of the ability to generate hypotheses and employ strategies and the ability to process the information Ss gain from their own responding is suggested to account for the major developmental differences reported.

In segregating F(2), F(3), and F(4) generations, albino mice had lower activity and higher defecation scores than pigmented animals when tested in a brightly lighted open field. These differences persisted when members of an F(5) generation were tested under white light, but largely disappeared under red light. Thus it was concluded that there is a major gene effect on the quantitative traits of open-field activity and defecation which is mediated by the visual system and that albino mice are more photophobic than pigmented mice under conditions of bright illumination.

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In mice: Effect of testThe open-field behavior of two inbred strains of mice (BALBjc! and C57BLj6!) C57BL/6J mice (a pigmented strain) have higher activity and lower defecation scores than BALB/cJ mice (albino) when tested in the open field under conditions of moderately bright illumination (approximately 40 ft-c). In segregating F2, F3, and F4 generations, pigmented animals have also been found to have relatively higher activity and lower defecation scores than albinos, indicating that approximately 20% of the difference between the original inbred strains in both open-field activity and defecation was due to a single-gene difference at the albino locus (DeFries et al, 1966). These differences persisted when animals of an F5 generation were tested under white light, but largely disappeared when tested under red light; therefore, it was suggested that albino mice are more photophobic than pigmented animals.If differences in open-field behavior are due in part to a visually mediated fear reaction which is a function of illumination, then members of both inbred strains should manifest increased "emotionality" (higher defecation and lower activity) as level of test illumination is increased. However, if albinos are more photophobic than pigmented animals, members of the BALB/cJ strain should be more affected than those of the C57BL/6J strain by changes in test illumination, at least at the lower levels. In order to investigate these possibilities, the open-field behavior of these two inbred strains of mice was observed under five levels of illumination (0 to 480 ft-c) in approximately equal log steps. MethodMembers of two inbred strains of mice (BALB/cJ and C57BL/6J) were mated within strains at 70± 10 days of age and the resulting inbred offspring were tested at 40± 2 days of age in a square (24 x 24 x 8 in.) open field. The floor and sides of the field were of white, painted Plexiglas. Two sets of five light sources were beamed through holes and red