We have sought evidence for genetic influences that are shared between addiction and 1) frontal lobe brain volumes, 2) cognitive function and 3) Alzheimer’s disease. Hypotheses about such shared genetic influences are based, in part, on initial observtions that so many of the genes that we and others have identified in addiction genome wide association relate to cell connections. These molecularly-based hypotheses were reinforced by the evidence for substantial, complex genetic components to each of these phenotypes. These hypotheses were strengthened by evidence, though often from small samples, that appears to document 1) small frontal lobe volumes in samples of addicts [19, 290], 2) lower performance levels on tests of cognitive and executive function in samples of addicts [291–293], and 3) large roles of heritability vs little role for the drug exposure itself in determining the cognitive abilities of twin pair members who are discordant for cannabis use [294]. These hypotheses are further reinforced by 1) twin data that document strong shared genetic influences on frontal brain volumes and cognitive function measures [246, 247], 2) smaller head (and thus likely
