A large-scale study on the development of attention-deficit/hyperactivity disorder (ADHD) finds the following: The likelihood of both fraternal twins (who share about 50% of their genes) having the disorder is significantly lower than for identical twins (who share 100% of their genes). Furthermore, the likelihood for fraternal twins is only slightly higher than the likelihood of two non-related adoptive siblings raised in the same home both having the disorder. What is the most logical conclusion that can be drawn from these findings regarding the causes of the disorder?

A large-scale study follows two groups of children from birth: Group A consists of children raised by their biological parents, and Group B consists of children who were adopted at birth and raised by genetically unrelated parents. The study finds that the prevalence of a specific neurodevelopmental disorder is virtually identical between both groups. What is the most logical conclusion that can be drawn from this finding regarding the development of the disorder?