We evaluate the extent to which changes in phenotypic variation among generations of populations kept in the same environment are due to changes in genetic (V-A) or in environmental (V-E) variance. Data were available on body weight of adult poultry on a total of 89186 birds (mainly females) from six generations of each of seven lines of layers. There was substantial heterogeneity of variation between generations, shown to be in both V-A and V-E components. Based on the Akaike information criterion (AIC), the best fit was with both components changing, and a better fit was obtained if V-A/V-E (i.e. heritability) or V-E, rather than V-A, was assumed constant. In analyses of quantitative genetic data spanning environmental groups, attention should be paid to whether and how the variance components change among groups before undertaking detailed variance partition that may be sensitive to such changes.