BIOLOGY IN HUMAN AFFAIRS 



influenced by ordinary physical defects (such as adenoids 

 or malnutrition) or by moderate differences in regularity 

 of school attendance, in pedagogical skill of teachers, or 

 in the cultural level of parents. At the same time it is freely 

 admitted that extreme differences in regard to such advan- 

 tages and disadvantages may and probably do, in some 

 degree, invalidate the test result. Ultimately the influence 

 of these and other possible determinants of intelligence 

 will be measured by the test method. In the second place, 

 one must bear in mind that even the best tests are not 

 perfectly consistent, or reliable. Every test score has its 

 probable error. This has been carefully ascertained for all 

 of the better types of tests. In the third place, one must 

 remember that no test samples every aspect of intelligence; 

 one may emphasize ability to deal with abstractions, 

 another the ability to adapt to common sense situations, 

 and another aptitude for working with things. 



A subject's test performances have little meaning until 

 they have been compared with norms. The norms that have 

 proved most useful are those based upon age. A subject 

 whose score equals that of an average six-year-old is said 

 to have a mental age level of six years; if the score equals 

 that of the average ten-year-old, the mental age is ten 

 years, etc. A particular subject whose actual chrono- 

 logical age is eight years may have a mental age of four, 

 six, eight, ten, twelve, or even fourteen years. Dividing 

 the mental age by the chronological age gives us an index 

 of brightness known as the intelligence quotient, or I.Q. 

 For example, the eight-year-old of six-year mental age 

 has an I.Q. of 75; the eight-year-old of ten-year mental 

 age, an I.Q. of 125. 



The educational significance of mental age lies in the 

 fact that it determines what a subject is able to learn, 

 what can be profitably taught him. Children entering 

 school with a mental age of six years can be taught to 



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