Abstract: Attempt was made to investigate the development of ability for the adequateconditional syllogistic reasoning with the inter-individual differences in school children.Three deductive syllogistic tasks with verbal materials were individually presented tothe subects at age 9, and at ages 12 and 15. The statements of each task containeddifferent propositions, all of which referred to the experimental contexts available toall school children. The results were shown that: (1)Three developmental levels of theability for the syllogistic reasoning were identifiedf Level 1(level of occrrence. Thechildren at mis level could only solve some parts of the tasks (such as reasoning theform of affirmation of antecedents). The syllogistic reasoning depended on thesustenance of the perceptual experiences; Level 2(transitional level). The ablities ofcombinatorial proposition and operational thenking were enhanced at this level ofchildren, however the syllogistic reasoning was frequently constrained by the concretecontexts of the tasks; Level 3(mature level). The ability of assumed thinking incoordination with the ability of deductive reasoning were developed. The "forms"were liberated from the contexts in this level. The syllogistic reasoning better accordedwith the logic rules as well. (2) The same subject and the subjects at different agesmastered four basic logic rules in relation with deductive reasoning investigated withdifferent sequences, which depended on the contexts and the natures of the tasks,also on the levels of operational thinking of the individuals. The developmental levelsof the subjects for syllogistic reasoning were affected by the qualities of schoolingand family education, also by the levels of intelligence of the subjects.

Key words: 9-15 year olds, conditional syllogistic reasoning, inter-individual differences