65 subjects (26 male; M age
22.4 yrs; SD age
: 4.9 yrs) completed the investment task and were subsequently genotyped for the 5-HTTLPR
functional polymorphisms. Genotyping was conducted by ACGT Inc. (Wheeling, IL) (see Supplementary Methods S1
All participants were affiliated with Northwestern University and were recruited via email announcements sent to the subject pool maintained by the Kellogg School of Management at Northwestern University. The experiment was programmed (and data were collected) using the software package E-Prime. Five to nine subjects were in the laboratory solving the investment task at the same time, with each individual in a cubicle separated from the rest of the participants. Participants gave informed consent prior to participating and the study was approved by the IRB committee at Northwestern University. The entire experiment took 1.5 hours to complete and the average pay per subject was $25.
The entire sample consisted of 21 carriers (8 male) homozygous for the s
allele, 44 carriers (18 male) with one or two copies of the l
allele of the 5-HTTLPR
polymorphism (see Supplementary Table S1
), 15 carriers (8 male) of the 7-repeat allele, and 50 non-carriers (18 male) of 7-repeat allele variant of DRD4
(see Supplementary Table S2
). We conducted statistical analyses to compare expected versus observed allele frequencies for our 5-HTTLPR
sample (see Supplementary Table S1
) and found that 5-HTTLPR
genotypes in our sample were distributed according to Hardy-Weinberg equilibrium (Pearson chi-square
>0.05). Additionally, we conducted statistical analyses for HW equilibrium of DRD4
genotypes (see Supplementary Table S2
) using the Markov Chain algorithm 
and found that DRD4
allele frequencies in the current sample were distributed according to HWE (Markov chain algorithm; p
Participants first completed the investment task and were then genotyped. On each of the 96 trials (see ) of the task subjects were given an amount of money $T. Subjects could invest $T+$15 (the show-up fee) in two assets, a riskless and a risky one. The amount not invested in the risky asset was automatically invested in the riskless asset (shorting and borrowing were not allowed). The trial endowment amount T was either $8 or $13, and hence the amount subjects could invest (T+$15 show-up fee) was either $23 or $28 per trial. In one version of the task, subjects were informed that the risky asset would pay either of two possible returns with equal probability, and these two possible outcomes for the return were known by the subject in each trial. In another version of the task, subjects were provided with the expected return and standard deviation of the risky asset. These two ways of presenting information about the payoffs of the risky investment are equivalent if subjects have mean-variance preferences (i.e. they like higher expected returns and lower variance), a common assumption in the finance literature which is also supported by our data. The riskless asset paid a known rate of return. Subjects' choices did not differ across these versions of the task (completed by 26 and 39 subjects, respectively), and therefore we combine the data from the two versions.
At the time of making a choice, subjects knew the actual rate of return of the risk-free asset and the two possible outcomes of the risky security, or, equivalently, the expected value and standard deviation of the risky return. These values differed from trial to trial, as did the amount of money available to the subject to invest. The actual rate of return for the risky asset on any trial was not revealed until the end of the experiment. At the end of the experiment, each subject selected a random number between 1 and 96 by picking a ball from an urn. That number determined the trial for which the subject would receive payment. If on any trial a subject chose to invest in the risky asset an amount larger that the maximum investment allowed ($T+$15), or if they did not respond, that trial was marked as invalid. If an invalid trial was selected from the urn, the final payment was only the show-up fee of $15. Subjects therefore had incentives to always enter their choice for the risky investment, and to treat each of the 96 trials as the one that would determine their pay. By deferring information about earnings until the end of the experiment we eliminate wealth effects that may change subjects' choices depending on past outcomes.
In each trial subjects had six seconds to learn the information about the return distribution of the risky security and the return of the safe asset. They had six additional seconds to enter the dollar amount they wished to invest in the risky asset, which was an integer that could range from zero to the maximum investment of $T+$15. A 2-second fixation screen indicated a new trial was about to begin.