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To separate age and cohort effects on decline in normal cognitive aging, we applied growth curve models to longitudinal data from the Swedish Adoption/Twin Study of Aging. Data from up to five measurement waves covering a 16-year period were available from 806 participants (age 50 to 88 at the first wave). We divided the sample into two cohorts by birth year: 1900-1925 and 1926-1948. We generated components to tap four cognitive domains: verbal and spatial ability, memory, and speed. We tested cohort differences by using two growth models: quadratic and two linear slopes. Results indicated significant cohort differences in average performance at age 67.5 for all components except speed. When we compared linear slopes during the same age range (age 62-78), we found no cohort differences. Trajectories of change with age in these four domains were fundamentally the same in middle-old age for individuals born during the first half of the 20th century.

Measures of memory performance and cognitive and lifestyle variables were obtained from 147 younger twin pairs (27 to 60 years) and 194 older twin pairs (60 to 94 years) as part of the Minnesota Twin Study of Adult Development and Aging. Correlational and behavior genetic analyses were combined to investigate age differences in 1) the relationship between memory performance and cognitive and lifestyle variables, 2) genetic and environmental influences on memory performance, and 3) genetic and environmental mediation of the relationship between memory and predictor variables. No age differences in heritability were found. Qualitative age differences in memory processes were suggested by age group differences in the correlations between memory and cognitive factors. Even though age differences in phenotypic correlations had been demonstrated, no age differences in the genetic and environmental mediation of the correlations were found. In other words, the magnitude of the correlation between memory and cognitive factors varied across age groups but the nature of the relationship did not.

Results from 2 twin studies of aging differ concerning the heritability of memory performance in adulthood. Heritability estimates from the Minnesota Twin Study of Adult Development and Aging (MTSADA; D. Finkel & M. McGue, 1993) are larger than estimates from the Swedish data (N. L. Pedersen, R. Plomin, J. R. Nesselroade, & G. E. McClearn, 1992). Memory data were available from MTSADA on a sample of 112 monozygotic (MZ) twin pairs and 111 dizygotic (DZ) twin pairs ages 27 to 88 years and from the Swedish study on a sample of 86 MZ and 110 DZ twin pairs who were reared together and were ages 27 to 85 years. Univariate and multivariate behavior genetic analyses were conducted to determine the significance and source of differences in heritability across studies. No significant age differences were found in either study. Study differences were found at the level of specific memory measures, but not at the level of a general memory factor.

A hypermnesic task was administered to 82 younger adults (ages 27-39), 63 middle-aged adults (ages 40-59), and 119 older adults (ages 60-87). Previous research suggests that relational encoding prevents loss of items and item-specific encoding promotes item gains in a hypermnesic task (Klein et al., 1989) and that there are age differences in relational but not item-specific encoding (Luszcz et al., 1990). This information provided the basis for three predictions: (a) There are age differences in hypermnesia, (b) there are age differences in word losses in a hypermnesic task, and (c) there are no age differences in word gains in a hypermnesic task. In order to manipulate type of encoding, a list of words with high association strength (to evoke relational encoding) and words with low association strength (to evoke item-specific encoding) was constructed. The results of this investigation provide support for the encoding manipulation and for all three predictions. In addition, the nature of the age differences in word loss observed suggests that although older adults may be capable of relational encoding, this form of encoding is not as effective at preventing word loss for them as it is for younger adults.

The purpose of this investigation was to apply behavior genetic methods to investigate individual differences in memory performance. Memory and various cognitive and lifestyle variables were obtained from 93 monozygotic twin pairs and 67 dizygotic twin pairs aged 60-88 years as part of the Minnesota Twin Study of Adult Development and Aging. Univariate analysis, used to determine the relative influence of genetic and environmental factors on 4 measures of memory (word recall, immediate and delayed text recall, and figure memory), suggested that 55% of the variance in memory performance could be attributed to genetic factors. Bivariate analysis was used to determine the specific variables that mediate the genetic and environmental influences on memory. Results suggested that the relationship between memory and cognitive variables was genetic in nature, whereas the nature of the relationship between memory and lifestyle variables was environmental.