

Intelligence and Emotion in Ageing 171 



environment. In the studies of humans, however, environ- 

 mental controls are impossible. The very circumstances under 

 which humans live introduce factors making for variations in 

 behaviour. Indeed, it has been suggested that the age of the 

 parents may be a factor in producing a different environment 

 for subgroups of children. Among humans, there must be a 

 reciprocal relationship between the effect of adult behaviour 

 on child behaviour (Riess, 1954; Scott and Marston, 1950). 

 Additional sources of variability are introduced by sampling 

 different subcultures within a geographic unit, or by sampling 

 at the same time different cultures. The socio-economic 

 environment of today must affect the behavioural growth, 

 development and decline, differently from the setting of the 

 Renaissance or the circumstances of the ancient Egyptian 

 dynasties. Different cultures at different times differed in 

 their w r ays of caring for the young and the old, of inducting 

 their children into their society, of working to produce food 

 and services and of maintaining the social values and mores. 

 The very setting — social, economic, vocational — must have 

 affected ageing not only in physiology but also in behaviour. 

 On account of the difficulty of controlling the environment 

 of humans, most of the evidence about the relation of be- 

 haviour to age has been obtained by the "cross- sectional" 

 method. Essentially, the cross-sectional approach, at its best, 

 studies some trait (or series of traits) in more or less repre- 

 sentative samples of specified age-groups in a population, 

 where age usually means chronological age. The desideratum 

 has been to select at each age a representative (and suf- 

 ficiently large) sample of five-year-olds, ten-year-olds, etc. 

 Galton's pioneer measurement of hearing, seeing, reaction 

 time, sense of perpendicularity and judgment (Ruger and 

 Stoesigger, 1927) and Jones and Conrad's (1933) evidence 

 about the growth and decline of intelligence were made by 

 the cross-sectional method. The basic assumption is that 

 samples of successive age groups are equivalent except for the 

 changes which age brings. The assumption, however, is not 

 consonant with the facts, for it should be obvious that 



