Decline in genetic influence on the co-occurrence of alcohol, marijuana, and nicotine dependence symptoms from age 14 to 29.

Mean Change in Symptom Count with Age. Females are displayed on the left and males on the right. Each substance is plotted in a different color. Means for Nicotine Dependence, Alcohol Dependence, and Marijuana Dependence are in red, black, and green, respectively. Error bars represent 95% confidence intervals.

Within and Across Age Correlations between Substance Use Symptom Count Measures. Males are reported in the lower triangle and females in the upper triangle. Correlations (without decimals) are displayed within each colored box. To aid visualization, the matrix is a heat map, with hotter colors signifying higher correlations. The matrix is organized into blocks by age. Note the trend in the bolded diagonal blocks; the colors generally become cooler as one moves from the upper left to lower right, indicating a steady decrease in correlations among the substances over time. The off-diagonals have purposefully been partially obscured to focus the reader’s attention on the block diagonal without omitting relevant information about the cross-age correlations. Females in the younger cohort had just begun their age-29 assessment, and thus the age-14/age-29 block is empty.

Percent of Symptom Count Variance Accounted for by the General Factor at Each Age. The grey lines in the figure show the decline in correlations over time as expressed by decline in the average percentage of variance accounted for by a general factor at each time point (i.e., the “Mean Squared Loading” column from Table 3). The full sample is shown on the left while the subsample of individuals who had atleast one nicotine, alcohol, or marijuana symptom by their age-17 assessment is shown on the right. The decline for both sexes in both samples was statistically significant (see Results). Sample sizes are given in the text as well as in Table 2. In the full male and female samples the blue, green, and purple lines represent the proportions of the general factor phenotypic variance (grey lines) that are due to genetic (blue), shared environmental (purple), and non-shared environmental (green) variance. Male values are always represented by the darker hue. These estimates can be computed directly from values provided in Table 3 by multiplying the mean squared loading by the corresponding ACE value (e.g., by the A value to obtain the additive genetic variance which is plotted in blue). The majority of phenotypic decline in the full sample is due to a statistically significant decline in heritability, as well as a non-statistically significant decline in shared environmental variance. In contrast, non-shared environmental variance significantly increases with age. Note that age-14 estimates are not given because girls at this age had few symptoms (see Results). Genetic and environmental components are not given for the subsample as it was composed of selected individuals, and not selected twin pairs (see Methods).