Mental Aspects of Ageing 37 



the measuring' instrument was measuring the same abilities 

 at different ages. 



When does dechne begin? Earher work seemed to show 

 that the average scores obtained on intelhgence tests dimin- 

 ished at an increasing rate from the third decade onwards. 

 But this is now questionable. Patrick Slater, on a carefully 

 selected and stratified sample of 2,500 men aged 18 to 42, 

 found no evidence of an increased rate of decline from thirty 

 onwards. Vincent, more recently, examined the scores of 

 7,297 subjects on a verbal intelligence test and found that 

 between the ages of twenty and sixty the mean score declined 

 steadily, showing a linear regression. The annual decline of 

 the mean score was found to be likewise constant when other 

 tests had been used: it was between -026 and -030 of the 

 standard deviation of the test. Such a finding, though 

 important, is less valuable than one which would establish 

 whether the annual decline in an individual is related to the 

 maximal level attained, say, at sixteen or twenty, or to the 

 rate of increment during the earlier years of life. As far as 

 I know there is no evidence at all on these points. 



A score on an intelligence test is a crude summation. If, 

 as seems to be the case, the decline of some abilities is more 

 rapid than that of others, the decrement will be small on some 

 subtests and large on others. The abilities least affected have 

 been those concerned with vocabulary and information, 

 while most rapid decline was evident in the ability to deduce 

 relations and adapt to new situations. A mean total score 

 would conceal these differences, as well as the differences 

 between individuals. The long dispute about a general factor 

 and special factors is relevant to this. Balinsky, using 

 Thurstone's method of factor analysis and intercorrelating 

 scores on subtests of the Wechsler Bellevue Scale at different 

 ages, concluded that mental organisation changes over a span 

 of years. From childhood onwards there is increasing special- 

 isation, and then later the various special abilities are inte- 

 grated into a flexible whole, but with subsequent changes in 

 the relative extent to which special abilities are utilized. No 



