Using the method of pointing out the main ideas of short articles, the author has found the development curve of the manner of thinking, which was marked by a conspicuous change at about 12 years of age. The number of children ranged from 54 to 71 each age group. He has also given a mathematical model of this psychic development curve, which resembles the mathematical model of the organic development curve of G. Backman and F. Hercik. But unlike their models, this one represents a superposition of two logarithmic functions. The author disagrees with those who use intelligence development curves, which are continuous and without conspicuous leaps, to prove the non-existence of development leaps in the manner of thinking. Intelligence development curves, which result from testing children’s intelligence, are based on the number of correct solutions in each age group. Therefore the development of thinking cannot be ascertained on the basis of such results. Using the dispersion analysis according to N. A. Plochinski, the author has found a statistically important share of the age factor. Pupils solved problems with action factors more successfully than problems with descriptive factors. It was not until about the age of 15 years that the difference in difficulty was levelled. There appeared to be certain differences between, boys and girls. Where boys show an increase in ability; girls show stagnation, and the other way about. But these differences were not statistically important. The author has found out the following stages in the development of thinking;
1. No generalization. Giving the main points of an article, pupils use random notions and sentences without any inner connection with the theme in question. This stage culminates at about 9 years of age.
2. Excessive generalization. It culminates at the age of about 13 years.
3. Correct generalization. This stage culminates at about 15 years of age, or sometimes later.