While developmental effects are clearly observable in the TICs, the effect of sex on substance parameters appears more subtle at the level of the whole dimension (see Figure 2), with information peaks being located at slightly higher trait levels in females than in males. This means that a female endorsing the same substance use pattern as a male is displaying a slightly greater severity level of poly-substance involvement. This is consistent with applications of IRT models to adult poly-substance involvement (Kirisci et al., 2002) and substance disorder severity (Kirisci et al., 2006), which also indicated greater severity in females than in males who endorse using or meet disorder criteria for the same substance(s). The current study extended these findings to show that sex differences in the poly-substance involvement dimension emerge early and remain constant throughout adolescence.
