fei75766_ch15_463-494.indd 484  02/24/20 10:48 AM
Part V    Biological/Evolutionary Theories484
isasearch for“genetic markers” ofbehavior. Thetraits arequantitative because they
aremarkers forbehaviors thatareexpressed onabroad continuum, from verylittleto
verymuch. Forexample, anxiety isaquantitative traitbecause some people arenot
atallanxious, most people areaverage, andafewareveryanxious. TheQTL method 
uncovers thelocation onparticular genes thatisassociated withhighorlowlevels of
atrait.These locations arealsoknown as“markers.”
QTL research points togenetic markers forseveral basic personality traits, 
suchasnovelty- orthrill seeking, impulsivity, andneuroticism/anxiety  (Benjamin 
etal.,1996; Dina etal.,2005; Hamer &Copeland, 1998; Lesch etal.,1996; Plomin &
Caspi, 1999; Retzetal.,2010; Rutter, 2006). Consider thecaseofthrill seeking, 
atraitthatentails risktaking. People withthistraitmayseekouthighly exciting 
activities likebungee jumping, mountain climbing, orscuba diving. Thrill-seeking 
activities create a“rush” ofexcitement—a positive feeling thatmayberelated tothe
release ofdopamine, aneurotransmitter associated withphysiological arousal. Given 
thepossible connection between dopamine andthrill seeking, onetheory suggests 
thatpeople whoaredeficient indopamine willtendtoseekoutexciting situations 
asawayofincreasing theirdopamine release andmaking upfordeficient levels of
dopamine.
Inthemid-1990s, researchers presented thefirstgenetic evidence tosupport 
thistheory. ThegeneDRD4 isinvolved indopamine production inthelimbic system, 
andthelonger thegenesequence, thelessefficient dopamine production is.Inother 
words, longversions oftheDRD4 geneareassociated withlessefficient dopamine 
production. Ifthetheory iscorrect, people whoseekoutthrills andnovel experi -
ences should havethelonger form ofthisgene, andthatisexactly what theresearch 
hasshown (Ebstein etal.,1996; Hamer &Copeland, 1998). Infact,consistent with
human research, DRD4 differences alsoaffect exploratory behavior likecuriosity and
seeking novel experiences innonhuman animals suchasbirds (Verhulst etal.,2016), 
apes(Shimada etal.,2004), anddogs(Itoetal.,2004). Anexciting aspect ofthis
finding isthatitwasthefirsttodemonstrate aspecific genetic influence onanormal 
(nonpathological) personality trait.
Thesecond method usedbybehavioral geneticists foruntangling theeffects of
genetics andenvironment onpersonality wasfirstdiscussed inChapter 14(Eysenck), 
namely twin-adoption studies .Recall thattheconclusion from thisresearch isthat
between 40%and60%ofourpersonality differences stem from genetic influence 
(Bouchard &Loehlin, 2001; Caspi, Roberts, &Shiner, 2003; Krueger &Johnson, 
2008; Loehlin etal.,1998; Plomin &Caspi, 1999; South, Reichborn-Kjennerud, 
Eaton, &Krueger, 2013; Vukasović &Bratko, 2015). Thislineofresearch therefore 
suggests thatabout halfofthedifferences thatexistbetween people intheirperson -
alities isattributable togenetics, leaving theother halfinfluenced byenvironment or
other unknown factors. Interestingly, personality disorders suchasavoidant personal -
ityandobsessive–compulsive personality alsoappear tobeprimarily under genetic 
influence. Gjerde andcolleagues (2015), forexample, reported 67%ofdifferences in
avoidant personality and53%ofdifferences inobsessive–compulsive personality can
beexplained bygenetic factors. These results areconsistent withtheviewthatperson -
ality, intelligence, motivation, andother psychological qualities areproducts notonly
ofbiological orenvironmental forces alone butalsooftheinteraction between the
two.Inshort, personality differences arecreated bybothnature andnurture.Final PDF to printer