Maturity as a cause of differences between individuals gives quite as unsatisfactory results as remote ancestry.  Every thoughtful student of children must realize that inner growth, apart from training, has something to do with the changes which take place in a child; that he differs from year to year because of a difference in maturity.  This same cause, then, must account to some extent for the differences between individuals of different ages.  But just how great a part it plays, what per cent of the difference it accounts for, and what particular traits it affects much or little, no one knows.  We say in general that nine-year-old children are more suggestible than six-year-old, and than fourteen-year-old; that the point of view of the fifteen-year-old is different from that of the eleven-year-old; that the power of sense discrimination gradually increases up to about sixteen, and so on.  That these facts are true, no one can question, but how far they are due to mere change in maturity and how far to training or to the increase in power of some particular capacity, such as understanding directions, or power of forced attention, is unknown.  The studies which have been undertaken along this line have failed in two particulars:  first, to distribute the actual changes found from year to year among the three possible causes, maturity, general powers of comprehension and the like, and training; second, to measure the same individuals from year to year.  This last error is very common in studies of human nature.  It is taken for granted that to examine ten year olds and then eleven year olds and then twelve year olds will give what ten year olds will become in one and two years’ time respectively.  To test a group of grammar grade children and then a group of high school and then a group of college students will not show the changes in maturity from grammar school to college.  The method is quite wrong, for it tests only the ten year olds that stay in school long enough to become twelve year olds; it measures only the very small per cent of the grammar school children who get to college.  In other words, it is measuring a more highly selected group and accepting the result obtained from them as true of the entire group.  Because of these two serious errors in the investigations our knowledge of the influence of maturity as a cause of individual differences is no better than opinion.  Two facts, however, such studies do make clear.  First, the supposition that “the increases in ability due to a given amount of progress toward maturity are closely alike for all children save the so-called ‘abnormally-precocious’ or ‘retarded’ is false.  The same fraction of the total inner development, from zero to adult ability, will produce very unequal results in different children.  Inner growth acts differently according to the original nature that is growing.  The notion that maturity is the main factor in the differences found amongst school children, so that grading and methods of teaching should be fitted closely to ‘stage of growth,’ is also false.  It is by no means very hard to find seven year olds who can do intellectual work in which one in twenty seventeen year olds would fail."[15]