116 Relationship of Lntelligence to Size and Shape of Head 
Taken as a whole, the intelligent group for both sexes appears to be more 
vai iable in age than the dull group ; but the differences are too slight to be given 
much weight. If we leave out the quick intelligent group, the difference still 
ajjpears, but is extremely slight*. We can only say that there possibly exists a 
small physical tendency for dullness to be concentrated more than intelligence on 
certain years of childhood. 
The matter of change of intelligence with age is so important that I have 
approached it from another standpoint. I have enquired what is the average 
intelligence at each age, instead of what is the average age of each grade of 
intelligence. This might seem the more reasonable method of approaching the 
problem. But the first method, since age is quantitative, admits of direct deter- 
mination of the means of the arrays ; in the second method we can only find the 
mean intelligence of each age group by assuming the previously discussed 
" normal " scale of intelligence. Still the matter is of such interest that it is 
worth reconsidering from this standpoint. I have accordingly determined the 
mean intelligence of each age group. This was done as follows : The ratio in 
which the mean divided the groups Intelligent and Slow Intelligent taken together 
was determined for each array on the basis of a normal distribution of intelligence. 
This group covei's on our scale a range of 180 mentaces. We are thus able to give 
the deviation from mediocrity of each age array in mentaces. This is exhibited in 
the following table : 
TABLE VI. 
Influence of Age on Intelligence in School Children. 
Boys 
Girls 
Age 
Group 
Division of Intelligent + 
Mentaces 
Division of Intelli- 
Mentaces 
Slow Intelligent Range 
from zero of 
gent + Slow Intelligent 
from zero of 
into two parts in ratio 
standard scale 
Eange 
standard scale 
3— 7 
50 to 50 
-10 
46 to 54 
+ 17 
8— 9 
62 to .38 
-12 
54 to 46 
+ 3 
10—11 
59 to 41 
- 6 
51 to 49 
+ 8 
12—13 
65 to 35 
-17 
56 to 44 
- 1 
H—15 
62 to 38 
-12 
61 to 39 
-10 
16—17 
66 to 34 
-19 
57 to 43 
- 3 
18—20 
59 to 41 
- 6 
55 to 45 
+ 1 
General 
Population 
62 to 38 
-12 
55 to 45 
+ 1 
Supposing we take 350 to 400 mentaces as the full mental equipment of the 
avei'age individual (see foot-note, p. Ill), it will be clear that these age variations 
are comparatively slight. It will not, however, do to consider them solely as 
variations of no account due to the chance deviations of random sampling. 
* Boys: Intelligent 2-819, Dull 2-784 ; Girls: Intelligent 2-982, Dull 2-975. 
