skewed distribution found at every age. In the paper all uses of the word “log” as short for “logarithm” mean natural logarithm, including all text on figures.) The mean of the developmental trajectory s(t) and its time derivative is estimated by a local linear regression calculation (Wu and Zhang, 2002; Wang, 2003; Hoover et al., 1998). The local linear regression calculation is a sequence of regression calculations each centered at a specific age in which the intercept of the regression line is the mean of the trajectory and the slope of the regression line the time derivative at the age at which the regression is centered. That is, a separate calculation is carried out to determine the mean and slope of the theta ERO power values for each central age, which ranges from ages 12 to 25 in one-tenth year intervals, using age-invariant covariates. Each calculation contains data extending before and after the central age (except for the very first and last in the sequence); models centered one year apart share at least 86% of their data and models two years apart share at least 75%, so that there is considerable overlap in the data used for calculation in nearby
