This is a first attempt to show how intelligence rises and falls throughout life. The rough graph below is an educated guess of the normal development; that is, the development in persons without severe brain damage or severe degenerative brain diseases:
In tabulated form:
|Age (yrs.)||Offset relative to adult level (I.Q. pts.)|
|- 3/4||-35 (moderate retardation) to -185 (exceptional intelligence)|
|16||-4 to -8|
|64||-2||This decline may differ per individual, depending on health and lifestyle|
The starting age is the moment just before fertilization of the egg, assuming that the new person's intelligence — in the sense of general ability — begins as soon as there is a one-celled embryo, and on the understanding that cell division and other such processes are abilities and therefore imply the existence of a, be it minute, amount of general ability. This is normally about nine months before birth, so at a negative age according to conventional methods for expressing age. The egg and sperm cell both possess intelligence too in this view, but do not belong to the new human. It can in fact be inferred from the above graph and table, knowing that the first cell division occurs 11 to 20 hours after fertilization, that the ability level of a one-celled human embryo 11 hours old lies in the order of a hundredth of an I.Q. point (ranging from .002 to .013, depending on future adult level).
Although a statistically derived I.Q. scale as commonly used has no true and absolute zero, at this starting point intelligence is guaranteed to be absolutely zero. Combined with the known relation between I.Q. and real-life functioning, as well as between modal population I.Q. and civilizational level, we have the raw material for a true ratio scale of intelligence on our hands.
Given that all start at zero, the childhood curve of a highly intelligent person must be much steeper than that of a lowly intelligent person, while the exact shape of the childhood curve will also differ between individuals of the same future adult level. The part of the graph up to ages of about 16-19 aims to show the room there is for these childhood curves to vary. For instance, in the period up to age 16, the curve of one of later adult I.Q. 35 advances at 1.9 points per year in this model, while that of one of I.Q. 185 advances at 10.6 points per year. These huge differences in rate of advancement remain hidden when childhood I.Q. is expressed in the conventional way (age-based). For good understanding, the starting levels of -35 and -185 in the model refer to the same absolute level.
Although one has gone most of the way by the time one is 16, there is still a shallow increase from there to about 32, and this increase too is greater for highly intelligent than for lowly intelligent. The numbers in the table are an educated guess thereof which may be refined later.
The graph also illustrates how deceptive the age-based or age-corrected scores are that regular psychology's I.Q. tests tend to give; were it for such tests and scores, the curve would be a straight horizontal line throughout at 0 below adult level, and could not show the room for variation for childhood curves. This tells us that especially age-based childhood "I.Q."s are of limited informational value compared to adult I.Q.s; both the child's true current adult I.Q. level and the child's rate of advancement remain hidden, apart from the fact that one does not know what the individual curve of the child will be (e.g., is it a late-bloomer whose steepest increase is still to come, or an early-bloomer who will flatten off, or something in between?)
Concluding, it is clear that when considering the development of intelligence throughout life, one has no option than to think in terms of an absolute scale, independent of age and preferably of a non-statistical nature.