Researchers from McGill University and the Douglas Mental Health University Institute in Canada have found that a gene variant linked with an increased risk for childhood ADHD also is the same one linked with smoking during adulthood. Previous research has found that people with childhood ADHD are twice as likely to start smoking as adults than children without the condition, and this research may begin to explain why.

Researchers examined five different genetic variants, called single nucleotide polymorphisms (SNPs), which have been found to be linked with various smoking behaviors. One, for example, has been found to influence the age at which smoking starts, and another has been linked with how many cigarettes a smoker will go through a day. The scientists looked at the DNA of 454 children with ADHD, who were between the ages of six and 12, as well as their siblings and parents. The researchers also examined how severe each child's ADHD was, through assessment of their level of behavioral and emotional problems. Mothers of the children were also asked if they smoked; of the 394 women who answered the question, 171 women said that they had smoked during pregnancy, and 223 did not.

One of the SNPs linked with the amount of cigarettes a person smokes was also strongly linked to ADHD. The allele was passed down in equal measures whether or not the child's mother smoked during pregnancy. Children with the allele struggled more often to complete tasks that required greater amounts of brainpower and concentration, and were more likely to have behavioral problems at home or at school.

Ridha Joober, one of the study authors, said to TIME magazine, "This may also explain why children with ADHD are more likely to be born to mothers who smoked during pregnancy. This genetic variant-and there are probably many others-increases the risk for smoking in mothers and the risk for ADHD in their children, suggesting that smoking during pregnancy and ADHD are, at least in part, due to these shared genetic determinants."

Though the study had a large sample size, researchers are careful to note that the genetic link warrants further studies.

The study was published in the journal Archives of Disease in Childhood.