The Maturity number is one of the later additions to the numerological chart, combining the two core numbers rather than being computed independently, and it belongs to the mid-twentieth-century elaboration of the American system rather than to its earliest form. Its logic is that the self of the name and the path of the birth date, distinct in youth, fuse into a single mature aim; the age at which the number is said to take effect varies between writers, commonly given as the late thirties or the fiftieth year. Like the rest of the system it rests on the Latin spelling of the name and the conventions of modern date-reckoning.
Walter Burkert's study of the Pythagorean tradition finds no ancient precedent for combining two already-modern figures, the Life Path and the Expression, into a third; Underwood Dudley's critical survey of the system treats this kind of derived, second-generation number as characteristic of how the American chart kept growing across the twentieth century, each successive writer adding new figures built from the ones before rather than returning to any older source. The sense that an Achievement reading names a real, emerging life purpose is also the kind of effect the psychologist Bertram Forer demonstrated in 1948, when identical, deliberately vague personality sketches handed to thirty-nine students were rated, on average, better than four out of five for personal accuracy, a result skeptics cite as the likely explanation for why a late-life aim stated this generally feels specifically earned.