A new study published in Nature Neuroscience shows that the genetic signature for the tendency to undervalue future rewards—known as delay discounting—may overlap with attention-deficit/hyperactivity disorder (ADHD), smoking, and weight.
The authors from the University of California San Diego School of Medicine studied data from 23andMe, a commercial genetic testing and analysis service. Data were obtained from customers who consented to participate in research and answer survey questions that assessed delay discounting. More than 23,000 people participants agreed to the study.
Results showed that approximately 12% of a person’s variation in delay discounting can be attributed to numerous genetic variants that also influence other psychiatric and behavioral traits.
“Studying the genetic basis of delay discounting is something I’ve wanted to do for the entirety of my 20 years of research, but it takes a huge number of people for a genetics study to be meaningful,” said senior author Abraham Palmer, PhD, professor of psychiatry and vice chair for basic research at UC San Diego School of Medicine. “By collaborating with a company that already has the genotypes for millions of people, all we needed was for them to answer a few questions. It would have been difficult to enroll and genotype this many research participants on our own in academia—it would’ve taken years and been cost prohibitive. This is a new model for science.”
Dr. Palmer explained that every complicated nervous system needs a way of assessing the value of current vs delayed rewards.
“A person’s ability to delay gratification is not just a curiosity, it’s integrally important to physical and mental health,” he said. “In addition, a person’s economic success is tied to delay discounting. Take seeking higher education and saving for retirement as examples—these future rewards are valuable in today’s economy, but we’re finding that not everyone has the same inclination to achieve them.”
The survey questions were used to assess delay discounting with questions such as, ““Would you rather have $55 today or $75 in 61 Days?” By comparing participants’ survey responses to their corresponding genotypes and complementary data from other studies, Dr. Palmer’s team found a number of genetic correlations.
“We discovered, for the first time, a genetic correlation between ADHD and delay discounting,” said first author Sandra Sanchez-Roige, PhD, a postdoctoral researcher in Dr. Palmer’s lab. “People with ADHD place less value in delayed rewards. That doesn’t mean that everyone with ADHD will undervalue future rewards or vice versa, just that the two factors have a common underlying genetic cause.”
The researchers also found that delay discounting is genetically correlated with smoking initiation. In other words, people who undervalue future rewards may be more likely to start smoking and less likely to quit once they begin. Body mass index (BMI) was also strongly correlated with delay discounting, suggesting that people who don’t place a high value on future rewards tend to have a higher BMI.
The team determined that delay discounting negatively correlated with three cognitive measures: college attainment, years of education, and childhood IQ. The genetic factors that predict delay discounting also predict these outcomes.
One shortcoming in studies that rely on surveys is the chance that some participants answered randomly or carelessly. Dr. Palmer’s survey included three questions to assess how carefully the research participants were answering questions. For example, one asked, “Would you rather have $60 today or $20 today?” Only 2.1% of participants answered “incorrectly” on these questions, assuring the researchers that the vast majority were answering the questions carefully.
“We are very thankful to the 23andMe research participants who took the time to complete our survey—they weren’t paid to do it, they are citizen-scientists volunteering their help,” Dr. Palmer said. “It’s quite a feeling to think that so many people were willing to help out in this interest of ours.”
Dr. Palmer hopes to expand the study to a larger and more diverse population to strengthen the findings.
“An even larger study would help start identifying specific genes with a higher level of confidence,” he said. “Then we can do hypothesis-driven studies of this trait with animal or cellular models.”
This research was funded, in part, by the Peter Boris Chair in Addictions Research at McMaster University/St. Joseph’s Healthcare Hamilton, Frontiers of Innovation Scholars Program, and Interdisciplinary Research Fellowship in NeuroAIDS.
To read more about this study, click here.