A study found that ambidextrous children (those that are both left- and right-handed) are more likely to develop ADHD symptoms later in life, compared to their left-handed and right-handed peers.
The article states:
"Mixed-handed children, relative to right-handed, had approximately a twofold increase in odds of having difficulties with language and scholastic performance at the age of 8 years. Eight years later, as 16-year-olds, adolescents had twofold increase in odds concerning difficulties in school with language and with ADHD symptoms" (Rodriguez, et al., 2010).
Study subjects were from the Northern Finland Birth Cohort 1986, which consisted of close to 8,000 children. The children were assessed at 7 years old and at 16 years old. Language difficulties, school performance, and mental health symptoms were examined using reports from teachers, parents, and the adolescents themselves.
When I originally posted the link to this study on Twitter, AureliaCotta made the point that "once they adjusted for gender, gestational age and weight, statistical significance was lost". Thank you, Aurelia!
Rodriguez A, et al. (2010) "Mixed-handedness is linked to mental health problems in children and adolescents". Pediatrics (doi:10.1542/peds.2009-1165).