188 SECTIONAL ADDRESSES 
intellectual grasp of a problem at an earlier age than the average child 
were bright or intelligent, and that those who were slow in doing so were 
dull ; and subsequent inquiry has shown that his assumption was well 
grounded. ‘The danger here lies in variations of opportunity and training. 
Obviously, a child who has not had the opportunity of using the current 
coinage, or of buying and selling (or playing at buying and selling), or of 
learning to read and write, is at a disadvantage when he is put through 
certain of the Binet tests. This danger, however, is not so serious as it 
appears at first sight, for the social environment of children living in 
civilised communities differs very little in so far as it affects the results of 
the tests, and most of the tests have been chosen so as to minimise the 
influence of the environmental factor. ‘These tests have been analysed 
and improved, and Spearman claims to have shown that they measure a 
central common factor which is intellectual in nature and which, to be 
non-committal and to avoid the ambiguities of everyday speech, he calls, 
not intelligence, but ‘ g.’ 
Mental tests have been used so extensively and in connection with so 
many problems that they have yielded information of social significance. 
They have been been applied more or less carefully, and in forms more or 
less satisfactory, to children of all ages, races and grades of society, and 
the results obtained raise some hope of getting reliable information 
regarding the distribution of intellect in the population as a whole and in 
the various professional, social and economic strata, and regarding its 
connection with fertility, disease, environment, and other conditions : 
they suggest too that at last we may have here a method of getting reliable 
information which will throw light on the puzzling problems of mental 
inheritance. 
Repeated application of these tests to the same children suggests that 
mental development, as measured by the tests, proceeds along lines 
analogous to those of physical development and that it reaches its maturity 
about the age of adolescence, as do stature and other physical characters. 
The rate of development is expressed by the ratio of the level reached by 
the individual to that reached by the average of his age—for example, a boy 
of age ten years who has reached only the level of the average nine-year-old 
is said to have an intelligence-ratio (mental-ratio or intelligence-quotient) 
of nine-tenths or go per cent. This figure seems to measure some innate 
capacity or capacities, for, though it varies from one person to another, yet it 
remains fairly constant for each individual and appears to be little affected 
by external circumstances. Even serious and long-continued spells of 
illness appear to affect it very little: it is only ailments producing pro- 
gressive deterioration of the central nervous system, especially of the 
brain, such as encephalitis lethargica and some forms of epilepsy, that 
reduce it. Absence from school may interfere with a child’s education 
and so promote social inefficiency without affecting his intelligence-ratio. 
Changes in social and physical environment have very little effect in 
modifying this ratio unless they be very great. Residence in an institution 
does not appear to make the ratios more alike than they were on admission, 
and children who have never seen their parents, but have been reared 
in the same homes, show the same differences of intellect as do their 
