Is age kinder to the initially more able?: Differential ageing of a verbal ability in the healthy old people in Edinburgh study

Studies that examine whether there is differential age-related decline in intelligence test scores according to initial cognitive status, social class and education are reviewed. We tested the differential ageing hypothesis which states that decline in ability scores over time will be lower in those with superior mental ability, more education and higher social class at the time of original testing. 387 healthy old people were tested at baseline and followed up 4 years later. The National Adult Reading Test (NART) was administered on both occasions. NART-estimated IQ fell by a mean of 2.1 points over 4 years (p < 0.001). Subjects in higher social class categories had higher NART scores overall (p < 0.001), and had smaller NART decrements over 4 years. Ex-professional and -secretarial groups fell by a mean of 1.4 points in this period, versus 3.2 points for skilled manual, semi-skilled and unskilled groups. In addition, those who fell more in NART were older (p = 0.001), were less educated (p < 0.001), and had lower NART (p < 0.001) scores at baseline. In summary, those with higher baseline ability, in higher social class groups, with more education and who are younger are relatively protected from decline in this verbal ability with age. Possible mechanisms of differential decline in verbal intelligence with age are discussed.