IV. RESULTADOS Y DISCUSIÓN
4.3. Desintegración familiar y conductas de los adolescentes de la I.E.S Inca
The impression that cognition declines steadily with age has emerged mainly from cross-sectional studies. Cross-sectional designs involve making ‘between-cohort contrasts’, i.e., cognitive performance is compared across different age-groups. However, there are methodological difficulties associated with this type of study, in particular, that which has become known as the ‘cohort effect’ (Stuart-Hamilton, 1991). This refers to the fact that differences between, for example, 20 year olds and 70 year olds are not confined merely to age, but include all those features of their upbringing and experience. Thus these two age-groups represent different cohorts, i.e., each group shares a common set of experiences which separate them from others preceding or following them m historical thne (Hayslip & Panek, 1993). This makes the drawing of any simple conclusions regarding the way in which cognitive fectors - such as intelligence - change with age, difficult.
For example, empirical findings have shown (e.g., Hayslip, Kennelly, & Maloy, 1990; Schaie & Labouvie-Vief, 1974) that later-bom cohorts appear to be intellectually advantaged when compared with earlier bom cohorts at the same ages. This phenomenon has been explained on the grounds that increased educational opportunities and improved
life-styles, including improved health care and better nutrition, have enabled successive generations to; "...reach ever higher ability asymptotes similar to the secular tends o f improvement fo r anthropometric and other biological markers" (Schaie, 1990, p. 297). Just how great is the discrepancy in results obtained from within-cohort analyses in comparison to data obtained from between-cohort analysis? In answer, some conclusions regarding the relative importance of the cohort effect are presented below.
One of the most comprehensive longitudinal examinations of the effects on intellectual abilities is the Seattle Longitudinal Study (Schaie, 1983; Schaie & Hertzog, 1986). This study involved testing over 5,000 subjects, at seven-year intervals, from ages 25 to 81 years. Throughout the investigation, subjects were assessed using Thurstone's Primary Mental Abilities Test (PMA) (Thurstone, 1938, Thurstone & Thurstone, 1949). This is a test batteiy made-up of a number of sub-tests, which, the authors claim, measure five distinct aspects of intelligence: spatial ability, reasoning, numerical ability, verbal meaning, and word fluency. The results of the study showed that, on average, there is gain in intelligence until the late 30's or early 40's then there is stability until the mid-50's or early 60's are reached. Average decrements from age 53 to 60 are quite small and are statistically significant only for number and word fluaicy. Once the age of 60 is reached, however, 7-year decrements are statistically significant throughout. The results indicate that average decline in psychological competence may begin for some as early as the mid - 50’s, but that it is typically of small magnitude until the 70’s are reached (Schaie, 1990).
The most important point about this study, however, is that both cross-sectional and longitudinal information were available, thus making it possible to evaluate the importance of critical cohort differences. However, Salthouse (1991) carried out a re-examination of the data (T-score values^) for three of the sub-tests, across the seven year period. By calculating the difference in the amount of change in selected aspects of cognitive performance in both longitudinal and cross-sectional data he was able to demonstrate that, in fact, very similar relations between age and cognitive-performance existed for both within (longitudinal) and mixed (cross-sectional) cohorts. Salthouse stated that despite only a limited sample of cognitive measures being examined (and given that adjustments were made for factors such as practice and selective attrition) there appeared to be little support for the argument that results from cross-sectional investigations are not truly representative because of confounds due to the cohort effect. He drew the following conclusion; ‘To the extent that the results o f Schaie’s Seattle Longitudinal Study provide an accurate reflection o f cognitive functioning in adulthood, therefore, these analyses suggest that cohort membership, per se, may have relatively little effect on either the direction or the magnitude o f age differences in cognitive performance ” (Salthouse, 1991, p. 112).
There are obvious benefits in employing a cross-sectional design over a longitudinal one, most notably an economy of time. To compare differences between 20 and 70 year-olds would take 50 years using a longitudinal method as opposed to a matter of weeks or months - depending on the sample size - for a cross-sectional study. However, such a study
^ According to Salthouse (1991), The T-score values ’’indicate the magnitude o f change inferred or observed across an interval o f 7 years, and thus roughly correspond to the slope o f the function relation age to performance” (p. 109).
is employed at the cost of the researcher not being absolutely sure how much of an age group difference is caused by aging per se, and how much is due to the influence of a whole range of environmental and social factors.
Powell (1994) avows that the cross-sectional method is an acceptable way of obtaining data for psychological investigations with older adults, but warns that comparing results derived fi'om two groups of different ages may exaggerate both how soon the decline occurs and the extent of the loss. He maintains, however, that there is no reason why age-related changes in cognition, recorded cross-sectionally, should not be generalizable, over time, to the same subjects.