"E. L. Earle ('03) measured the spelling abilities of some 800 children in the St. Xavier school in New York by careful tests. As the children in this school commonly enter at a very early age, and as the staff and methods of teaching remain very constant, we have in the case of the 180 pairs of brothers and sisters included in the 600 children closely similar school training. Mr. Earle measured the ability of any individual by his deviation from the average for his grade and sex, and found the coefficient of correlation between children of the same family to be .50. That is, any individual is on the average 50% as much above or below the average for his age and sex as his brother or sister.
"Similarities of home training might account for this, but any one experienced in teaching will hesitate to attribute much efficacy to such similarities. Bad spellers remain bad spellers though their teachers change. Moreover, Dr. J. M. Rice in his exhaustive study of spelling ability ('97) found little or no relationship between good spelling and any one of the popular methods, and little or none between poor spelling and foreign parentage. Cornman's more careful study of spelling ('07) supports the view that ability to spell is little influenced by such differences in school or home training as commonly exist."
Este es un caso clarísimo de una capacidad intelectual definida, cuyas diferencias podrían suponerse debidas casi por completo a la educación del niño, pero que parecen, tras la investigación, deberse en gran medida a la herencia.
El problema puede examinarse con aún mayor detalle. ¿Hereda el hombre meramente la habilidad manual, digamos, o hereda el tipo preciso de habilidad manual necesario para hacer a un cirujano pero no el tipo que le sería útil a un relojero? ¿Nace el hombre meramente con una capacidad «artística» generalizada, o es esta una adaptada exclusivamente para, digamos, la música; o más aún, está adaptada exclusivamente para tocar el violín, y no para el piano?