This experimental study with a controlled pre-test and post-test group design investigated the possible effects of four sessions of heading training on cognitive performance. A total of 118 participants (mean age of 22.25 years) were assigned to one of the three following groups: a heading-training group, a passing-training group and a waiting control group. The participants completed a cognitive test battery of attention (D2-test), working memory (Paced Auditory Serial Addition Task, PASAT-2) and visual spatial intelligence (Mental Rotation Test) tests before, immediately after the second training session and one week after the fourth training session. There were no between-group differences in cognitive performance after the second or the fourth heading training sessions. However, within the heading-training group, women complained more of headaches than men after the fourth training session. These results build on the results from the study of Rieder and Jansen (2011) that showed no neuropsychological consequences after one heading session, showing no cumulative effects of repeat heading.

To search for an association of twin zygosity with tooth size, 56 dental variables measured from 65 pairs of twins (43 MZ, 22 DZ) were studied. Results of the t′ test for equality of the means showed no association of zygosity with any of the variables in males or in females. Results of the F′ test for homogeneity of total variances between zygosities showed evidence for unequal total variances in 15 variables in males and 13 in females. Sex influence was further noted on the association of zygosity with the variance of tooth size. Where total variances were unequal, genetic variance estimates differed when only the within-pair mean squares were used and when combined estimates designed to be unbiased by differences in environmental variances were used.