Five standard Piagetian tests were administered to 180 adolescents between the ages of 10 and 15 years. The results were compared with those obtained in 1967 and in 1972 for similar participant samples. At equal ages, today's adolescents exhibited a higher level of cognitive development than the adolescents of 20 or 30 years ago. The amount of gain observed varied across tasks, being very large for combinatory thought but mixed for conservation. This acceleration of cognitive development can partially explain the continuous rise in intelligence test performance (Flynn effect). Many cohort comparisons have shown that the mean score on a variety of intelligence tests (e.g., Raven Progressive Matrices, Stanford-Binet Intelligence Scale, Wechsler Intelligence scales) has been rising by approximately 3 IQ points per decade for at least 60 years (see reviews by Flynn, 1984, 1987; Storfer, 1990). This rise has been noted in over 20 countries, including France, where the studies reported in the present article took place (e.g.,