We begin by reviewing the results from the linear spline models. On average, test-retest stability was relatively high (b0 = .554, p < .001). Genetic stability (b0 = .844, p < .001) was substantially greater than both environmental (b0 = .397, p < .001) and corrected environmental stability (b0 = .652, p < .001). However, stability differed substantially over the lifespan. For phenotypic stability, the full model indicated that in infancy differential stability was not different from zero (b0 = .093, p = .47). Stability rose sharply over the first three years of life (b0–3 = .126, p < .05), plateaued during childhood (b3–15 = .001, p = .88), and then resumed increasing substantially until age 30 (b15–30 = .016, p < .001). Following age 30, no slope was significantly different from zero, but there was a small trend toward decreasing stability in old age. The model comparison indicated that the slopes for ages 30–60 and 60–90 could be constrained to be equal, but every other slope was needed.
