Age-truncation Biases

The problem of age truncation (sampling a single age or a very restricted age group) occurs quite frequently in birth-order research on intelligence because young people are being sampled. For example, if one has sampled 11-year-olds, those who are the oldest and from large families must come from closely spaced families because all of the children in the sibsize must have been born during an 11-year period. The youngest 11-year-olds from large families have not necessarily come from such closely spaced sibsizes. Thus, it is not at all uncommon to find that, among youthful samples, there may be an upturn for later-born children in the dependent variable (say, in IQ), an upturn that cannot be interpreted out of hand as a birth-order effect. The upturn at later birth orders in IQ testing is a not-infrequent result among age-restricted samples of young people, as figures 5.3 through 5.5 illustrate.^[2]

We should emphasize here that in general very wide spacing in large families (six or seven or more siblings) is not possible. For example, women beginning childbearing at age 20 and having a child every five years would be limited to five children even assuming that their last child was born when the woman was 40. It is thus not practical to think of wide spacing in families of six or seven or more as a generally compensatory mechanism allowing parents to provide more attention to each child—although obviously an individual child, or even two children, can be widely spaced in an

Figure 5.3.
T-Scores on Group Test of Intelligence by Sibsize and Birth Order
among Scottish School Children Age 11 in 1947. Data from the
Scottish Council for Research in Education (1949). Scores have been
restandardized with a mean of 50 and standard deviation of 10.

otherwise closely spaced family. Although extremes in spacing are uncommon, it would be desirable to know more about spacing in data sets such as we have been using because such information would provide an important test of the dilution hypothesis. Other things equal, we would expect widely spaced youngsters to have characteristics associated with smaller family size.