Among one group of 178 kids that has passed the threshold of adolescence, Tremblay checked out hormone-behavior links by measuring the boys's levels of testosterone. What he found set him on his ear. The boys who were rated (by peers and teachers) most physically aggressive at ages six to 12 had lower testosterone levels at age 13 than ordinary peers. The "multiproblem fighters," or hothead bullies, proved to have the lowest testosterone levels of all.