The association of DRD4 and 5-HTTLPR with infant temperament

We and others have previously shown that the dopamine D4
exon III repeat (D4DR) and the serotonin-transporter promoter
region (5-HTTLPR) polymorphisms are not only associated with
adult personality traits but also with temperament in 2-week old
neonates. We now report the results of a third study of these infants
and their temperament at 12 months. This study examined the
association between two common polymorphisms, the dopamine
D4 receptor (DRD4) gene and the serotonin transporter promoter (5-HTTLPR) and temperament in sixty-one 12-month-old infants.
Twenty-two infants had a least one copy of the 6-8 repeat DRD4
alleles (L-DRD4) and 39 had two copies of the 2-5 repeat allele
(S-DRD4). Twenty infants were homozygous for the short form
(s/s) of 5-HTTLPR while 41 were either heterozygous for the
short and the long form (l/s) or were homozygous for the long
form (1/1). The infants were observed in a series of standard
temperament episodes that elicited fear, anger, pleasure, interest,
and activity. L-DRD4 infants showed less interest in a structured
block play situation and more activity in a free play situation.
They also displayed less anger in an episode of mild physical
restraint. Infants with s/s 5-HTTLPR showed less fearful distress
to stranger approach and less pleasure in a structured play situation
than infants with 1/1 or 1/s 5-HTTLPR. Duration of looking during
block play was affected by a significant interaction between DRD4
and 5-HTTLPR. Shortest duration of looking was associated with
the L-DRD4 and s/s 5-HTTLPR genotypes. These results support
previous data showing that the origins of the molecular control of
human temperament lie in infancy.