516 INHERITANCE OF MENTAL TRAITS 



Variability in Other Traits. — Tests of musical 

 ability show similar ranges of differences for such traits 

 as discrimination of pitch, sense of rhythm, tonal mem- 

 ory, and appreciation of consonance; and that the varia- 

 tion in these traits also is continuous. It is probable 

 that if we could measure such traits as conscientiousness, 

 will power, sanity or social adaptability, we would find 

 the range of individual differences as great as for intel- 

 ligence and musical ability, and that the differences found 

 would be continuous. With mental traits it is rarely 

 a question of simple absence or presence, but rather of 

 presence in greater or less degree. The inference is that 

 such traits do not depend upon single factors in the 

 chromosomes, but upon several factors. Intellect, like 

 height, is perhaps in reality a composite of several more 

 elemental traits. Epilepsy and possibly a few other 

 forms of mental defect may be exceptions to this rule. 



Statistical Methods Used in the Measurement of 

 Resemblance, — Since the study of mental inheritance 

 involves a consideration of degrees of resemblance be- 

 tween parents and offspring, rather than the mere noting 

 of presence or absence of particular traits, certain statis- 

 tical devices have been found necessary. Among these 

 is the coefficient of correlation, which is a numerical in- 

 dex expressing the amount of agreement between two 

 sets of facts. 



For example, let us suppose that it is desired to express 

 the relationship existing between intelligence of fathers 

 and intelligence of sons, and that the intelligence of a 

 thousand father-son pairs has been measured by mental 

 tests. If the brightest father were found to have the 

 brightest son, the next-brightest father the next-bright- 

 est son, and so on down to the dullest, the correlation 

 coefficient would be + 1, which would represent complete 

 agreement between the two sets of facts. If the brightest 

 father were found to have the dullest son, the next- 

 brightest father the next-dullest son, etc., the correla- 



