The maturation lag model explains inattention and impulsivity in Attention Deficit Hyperactivity Disorder (ADHD) as delayed maturation along a normal developmental trajectory. The concept of a cross-culturally uniform developmental trajectory is tested by a comparison of the performance of 212 Mexican school children on the Test of Variable Attention (TOVA) with the performance of populations previously studied. An observed pattern of decreasing errors of omission (indicating improving ability to sustain attention) with increasing age did confirm the predictions of the existing developmental trajectory model, although the shape of this change was linear rather than curvilinear. A predicted age-related decrease in errors of commission (indicating improving impulse control) was not observed. Gender differences in attentional and impulse control measures among Mexican children, aged 6-12 years, were not significant, in contrast to the findings of previous US studies in which boys performed poorly compared with girls. Mexican children made significantly more errors of omission and commission than American children, indicating greater degrees of characteristic inattentive and impulsive behaviours in childhood. These results indicate that the assumption of a uniform developmental trajectory of these behaviours should be carefully considered before it is applied to understanding children's behaviour in culturally diverse settings.Extremely inattentive and impulsive behaviour in children is associated with Attention Deficit Hyperactivity Disorder (ADHD). However, impulsivity and inattention at lower levels are expected and common behaviour in most children, especially at younger ages. As they mature, children are believed to move along a trajectory toward increased attentional performance and impulse control, and these types of behaviour usually lessen or disappear with increasing age. The ''maturation lag'' theory proposes that the behaviour of children with ADHD may be explained by relative delays along this typical developmental trajectory for attention and impulse control, as compared with other children of the same age (