Given two protagonists in a pair of stories who do the same thing but one of them has external reasons such as reward and command for the behavior, the preschoolers tend to choose the protagonist with an external reason as having stronger internal motives for the behavior than the protagonist wlthout any clear external reason. The causes for this attributional error, termed the augmentation error, have been extensively investigated in the children`s development of causal attribution. So has been an issue regarding the relative onset time of the use of the discounting and the augmentation principles. When two plausible causes present for a given behavior and both are facilitatory in nature, people tend to discount the effect of one cause and accept the other, usually more salient one, as the cause of that behavior. The augmentation principle is believed to work at a different situation: When an inhibitory external cause is present for a given behavior, the protagonist having done that behavior is regarded as having strong internal motives for that behavior. That is, the presence of an inhibitory external cause contributes to augmentation of internal motives for a given behavior. A series of three experiments has been conducted to explore possible answers to two major issues: reasons for the augmentation error and the relative onset time of the preschoolers` use of the discounting and the augmentation principles. Sedlak and Kurtz (1981) proposed that for the preschoolers, the augmentation principle is employed earlier than the discounting principle in causal attribution. They also suggested two possible reasons for the augmentation error: the preschoolers` predominant tendency for cause-effect syncretism and failure of decentration of correlation between internal motives and external reward. Experiment I tested two hypotheses regarding the augmentation error. Under the condition in which the influences of the failure of correlation decentration were minimized as much as possible, preschoolers were given different types of feedback concerning two protagonits` behavioral effects. The subjects in the behavioral effect constancy condition were not informed of any differential effects of two protagonists. The subjects in the behavioral effect inconstancy condition were informed that the protagonist with an external cause enjoyed doing a certain thing whereas the protagonist without any external caused did not. More augmentation errors in the preschoolers` internal attribution were observed in the behavioral effect inconstancy condition than in the behavioral effect constancy condition. The six-year-old group showed more augmentation errors than the seven-year-old group. These results were interpreted as support for the cause-effect syncretism hypothesis. Experiment II was desigaed to explore the relative onset time of the use of the discounting and the augmentation principles in the preschooler group. The roles of behavioral effect constancy factor in their causal attribution were reexamined. The former issue was tested by manipulating the presence of inhibitory versus facilitatory external causes for a given behavior. The preschoolers of Experiment II were found to employ the augmentation principle earlier than the discounting principle in inferring two protagonists` internal motives. The influences of behavioral effect constancy on causal attribution were again confirmed. Experiment III was attempted to explore the relative importance of the behavioral effect constancy and the justification factors. Several of the previous studies indicated that preschoolers tended to show less augmentation errors when external causes were more clearly justified than they were not. Using a different sample of preschoolers, the main effect of the behavioral effect constancy was again confirmed while that of the justification factor was not. Taken together, results of these three experiments showed the robust influences of behavioral effect co