114 Relationship of Intelligence to Size and Shape of Head 
From these Tables the following constants were calculated : 
TABLE V. 
Vm^iahility and Mean Ages of Mental Groups. 
Group 
Boys 
Girls 
Mean Age 
Standard Deviation 
Mean Age 
Standard Deviation 
Quick Intelligent 
Intelligent 
Slow Intelligent... 
Slow 
Slow Dull 
Very Dull 
12-01 
12-48 
12-49 
12-46 
12-52 
12-50 
2-964 ) 
2-872 \ 2-867 
2-766 ) 
2-833 
2-755 > 2-784 
2-764 ) 
12-67 
12-45 
12- 64 
13- 17 
13-03 
13-27 
3-214 ) 
2-978 > 3-059 
2-985 ) 
2- 970 
3- 077 > 2-975 
2-878 ) 
Whole Population 
12-43 yrs 
2-839 yrs 
12 -67 yrs 
3-028 yrs 
Examining the means of each intelligence grade first, and taking the boys to 
start with, we note : That if we omit the quick intelligent group, the mean ages 
of each intelligence group are essentially the same. With the girls, dullness seems 
to increase somewhat with age. If we examine the broken vertical lines Fig. 2, 
we can see, I think, a differentiation between boys and girls ; the duller girls 
have a greater average age. Now the lesser age of the Quick Intelligent boys 
is, I think, due to the fact that bright children are allowed to go to school rather 
sooner than dull. But the differentiation between boys and girls is most probably 
due to the fact that the elder girls, 13 to 15, are commencing a period of life when 
physical demands upon them introduce very often a temporary and protective 
intellectual inertia. In the case of boys and girls, the influence of age on the 
extreme grades of intelligence scarcely amounts to six months at the most; and 
if we consider the facts that bright children go early to school, and leave early, 
while dull children go late and leave late, and again that the elder girls are 
especially apt to feel intellectually the burden of physical development, I think 
we may safely assert that there is no substantial change of intelligence with age. 
The actual correlation ratios are : 
for boys : = -054 ± "014 ; for girls : = "081+ -014 ; 
and these mark a sensible, but extremely slight, decrease of intelligence with age. 
This decrease is explicable on the grounds just referred to. 
We may consider here whether intelligence or dullness is the more scattered 
character. Turning to the columns of standard deviations, we notice : That for 
both girls and boys the maximum variability falls to the group of quick intelli- 
gence. This is probably due to the fact already noted, that the group is not 
so homogeneous as the other groups, containing a larger proportion of very able 
children sent young to school. 
