Nature vs. Nurture:
Behavioral Genetics and the Law
By Kathryn Baker
The Gene x Environment
Interaction
The formula for how genetics and life experiences
influence a person
Many professionals believe that violent behavior is
linked to a person’s life experiences
Research is being done on many fronts to help the
courts deal with new discoveries regarding the
human genome and how it affects behavior
What is MAOA?
Monoamine Oxidase A – a gene on the X
chromosome; codes for MAOA enzyme which,
when there are low amounts, inactivates
neurotransmitters like norepinephrine, serotonin,
and dopamine
Linked to the prevalence of anti-social and
aggressive behavior in males who were
maltreated as children
The Dunedin Study: Testing the
Effects of MAOA
Hypothesis: “that MAOA genotype can moderate
the influence of childhood maltreatment on neural
systems implicated in antisocial behavior”
Participants (3-11 years of age): 8% severely
maltreated as children, 28% probable
maltreatment, 64% no maltreatment
Psychological assessment at age 26
Summary of some results:
Males with high levels of MAOA activity were less
affected by childhood maltreatment
Conduct disorder more common among maltreated
males with low levels of MAOA activity
Violent crime convictions more common among
maltreated males with low levels of MAOA activity
The Numbers
Approximately 80% of maltreated males with low
levels of MAOA activity were reported to have
conduct disorder
Approximately 40% of maltreated males with high
levels of MAOA activity were reported to have
conduct disorder
Maltreated males with low levels of MAOA activity
made up 12% of the group BUT they accounted
for 44% of the group’s violent crime convictions
85% of maltreated males with low levels of MAOA
activity developed antisocial behaviors
Court Cases
Mobley v. State (1994): Mobley convicted of
murder, armed robbery, aggravated assault and
possession of a firearm; wanted evidence
presented that said he had deficient MAOA
activity; denied because it “lacked scientific
verifiability sufficient for it to be introduced;”
executed on March 1, 2005
State v. Sanders: the “serotonin defense;” low
levels of serotonin led to an inability to control
impulsive behavior; denied during trial because “It
could not determine that… he lacked the specific
intent to kill… only demonstrates that he was
enraged when he did;” accepted during sentencing
– sentenced to life without parole
Court Cases Continued
State v. Payne: expert testimony introduced about
Payne’s low levels of serotonin which suggested
that he could not “control [an] impulse once it had
occurred;” jury found Payne guilty of seconddegree rather than first-degree murder (Payne
was also under the influence of cocaine and
alcohol at the time of the crime)
What is SLC6A4?
The serotonin transport gene
Short allele causes low transport activity – the
serotonin system is inefficient
Low levels of activity could lead to depression in
some individuals
Genotyping Defendants
AA: High activity MAOA, short SLC6A4 allele;
expert testified AA had a “genetic vulnerability to
become depressed under severe stress;” found
guilty of first-degree murder
BB: High activity MAOA; long SLC6A4 allele;
expert testified that there were “no genetic factors”
that “might predispose BB to violence;” nobody
objected to this testimony
DD: High activity MAOA; heterozygous for
SLC6A4; court suggested that genetic testing
could become necessary component of defense
FF: High activity MAOA; short SLC6A4 allele;
court decided behavioral genetics was “not yet
scientific enough to present as testimony before a
jury”
Implications and Questions
Is this a sound defense against the death penalty:
A “person should not receive the death penalty
because his behavior was at least partly caused
by his genetic makeup and his adverse life
experiences”?
What about personal responsibility?
The redefinition of “normal.” Does anyone who is
“normal” commit a violent crime? Does every
criminal have a genetic disorder – will they in the
future?
Is evidence of genetic and behavioral interaction
acceptable for defending the insanity defense?
Not guilty by reason of insanity vs. Guilty but
mentally ill.
What types of punishment become acceptable?
Resources
Caspi, Avshalom et al. “Role of Genotype in the
Cycle of Violence in Maltreated Children.” Science
Vol. 297, 2002.
Bernet, William et al. “Bad Nature, Bad Nurture,
and Testimony regarding MAOA and SLC6A4
Genotyping at Murder Trials.” J Forensic Science
Vol. 52, No. 6, 2007.