There is a lack of consensus regarding the necessary and sufficient steps for empirically validating a measurement as a suitable candidate for an ADHD endophenotype.32–37 However, there is almost unanimous agreement on the following four criteria. One, the measurement needs to have good metric properties; scores should be continuously distributed without obvious floor or ceiling effects, they should show high internal consistency (the components that make up the measurement should correlate highly with each other), and test-retest reliability should be high. Two, it must be correlated with the disorder or its behavioral symptoms. Three, the measurement must be heritable. Four, at least a portion of this heritable variance is shared with the genetic liability for ADHD. This fourth criterion is traditionally examined by “means comparisons analysis” on groups of ADHD probands, unaffected siblings of the probands, and controls. If familial factors contribute to the endophenotype, then the means should be ordered as follows: controls ← unaffected siblings ← probands. This approach has been successful in other fields of neuropsychiatric research suc as schizophrenia.38,39 However, initial failures to substantiate measurements of
