The science of life : fully illustrated in tone and line and including many diagrams

BOOK 8

to fit square human pegs into round occupational holes and vice versa.

Using these intelligence tests, we can compare the performance of a child with average performance. We find that one child of ten attains the average for twelve-yearolds, while another is only up to the eightyear average. Both children. are ten years old ; but we can say that one has a “ mental age’ of twelve, the other a “‘ mental age” of eight. A child is generally classed as mentally deficient when its “‘ mental age,” determined by these tests, is less than threequarters of its chronological age in years. The actual capacity for being intelligent does not go on developing, even in the ablest men and women, after about fifteen or sixteen years of actual age. What increases after that is judgment and concrete knowledge, so that the intellectual edifice continues to be built, but not the pure faculty of intelligence by which it is built.

If we take a set of children and test their intelligence year by year, we find that the tests are consistent—a child is not below normal one year, above normal the next. What we are really measuring is the rate at which intelligence develops. And we find that intelligence behaves just as do

PER CENT 3

NUMBER OF CHILDREN

MENTAL RATIO 50 60 70 80 90

MENTALLY Dereeri DULL AND BACKWARD

Fig. 326.

THE SCIENCE OF LIFE

ORDINARY ELEMENTARY SCHOOL

CHAPTER 7

physical characters like the shrimp’s eyecolour which we discussed in Book 4 (Fig. 185); the intelligence of different children grows at different speeds, and stops growing at different ages. Professor Spearman has given strong reasons for believing that what is measured by the intelligence tests is some general factor which underlies the performance of any and every kind of mental operation demanding intelligence. He prefers to speak of it by the non-committal symbol of “g.” In popular parlance it could be called mental energy, but the word energy has a perfectly definite scientific meaning, and its use in this loose way should be avoided. ‘There is assuredly a flow of nerve-impulses which is the physical basis of thought ; and when we know more about the mechanism of this flow we shall be able to give Spearman’s ‘““ x? a more concrete interpretation. Meanwhile it must suffice to know that we can measure something which is the basis for general intelligence.

There is one other conclusion that we can draw; and that is that differences in this capacity for intelligence are due mainly to differences in hereditary make-up. As always, nature and nurture are blended.

10 120 130 140 150 MENTAL RATIO CENTRAL SECONDARY SCHOOL SCHOOL

The variation of intelligence in a group of 2,000 school children.

Carefully devised intelligence tests are applied on a large scale, and the average performance of children is found for

each age. This is taken as a standard (100).

If a child is ten years old and only attains the level of an average child

pre :, aie A 7 6 “ aoa: ro re 6 ° of seven or eight, he is said to have a *‘ mental ratio?’ of gy or 70 per cent.; children with mental ratio below this

are definitely defective. If he had done as well as the average child of 13, his mental ratio would be 130.

There

1s no break to separate the mental defective from the normal, or the normal from the genius. (From ‘‘ The Measurement

of Mental Capacities,’ by Dr. Cyril Burt.

822

Oliver G Boyd.)