Blame Your Bad Behavior On Your Parents: How ADHD, Obesity and Smoking Are Linked To Genes

For people who struggle to say "no" to another slice of pie or whose last cigarette is never quite their last, the fight may start in their genes. One part of human's ability to control their impulses might be rooted in variations of a set of genes, according to a paper published in Nature Neuroscience on Monday that used data collected by the genetic testing company 23andMe.

This is the first time that a particular trait called delay discounting has been linked with genes. Having a better understanding of how these genes influence this particular trait could lead to more precise diagnoses or treatments in the future for related conditions.

In essence, delay discounting is what’s happening when someone tends to always pick an immediate reward over a slightly larger reward further in the future. Delay discounting is part of impulsivity, and has been linked with ADHD, substance abuse disorders and weight problems.

Researchers already knew about the link between delay discounting and these conditions, researcher Abraham Palmer told Newsweek, but they didn’t know what might be driving that connection.

“It may be that we can discover risk factors for diseases like ADHD and drug abuse by studying delay discounting," said Palmer, a researcher at the University of California, San Diego's medical school. "It might be easier to study delay discounting with really large cohorts than it would be to study these disease diagnoses.”

Ritalin A bottle of Ritalin sits on the counter of the Post Haste Pharmacy And Surgical Store on June 16, 2003 in Hollywood, Florida. Joe Raedle/Getty Images

Palmer worked on the paper with Sandra Sanchez-Roige, a postdoctoral researcher in his lab, as well as with scientists at other universities and from 23andMe. People who had already ordered genetic sequencing through 23andMe and had given permission for data to be used for research purposes were asked to complete a survey related to delay discounting. 

One of the basic relationships of genetics is that a person’s genotype—defined by the specific genes in their body—contributes to their phenotype—the kind of traits they have. These traits are what help define us. Our hair color is a trait. So is our height. So is whether or not we have schizophrenia, Huntington’s disease or another condition with genetic links.

However, a person’s genotype does not determine their fate. "These traits are not 100 percent heritable," Sanchez-Roige emphasized. That's because a person’s environment can change how a gene may or may not lead to a trait. For some traits, the environment is more of a factor than it is for others. In the case of delay discounting, the common variants of a collection of genes can account for about 12 percent of the differences between people. 

This number may seem a bit low—after all, up to 80 percent of the likelihood that a person will or will not have schizophrenia is thought to be based in a person’s genetic code. However, Palmer noted, those kinds of studies look at both rare and common genetic variants; their work only looked at common ones.

“If we estimated schizophrenia risk using this common variance approach, we would get a lower number,” he explained.

That’s a relatively minor issue; common variants are, after all, common. But the population in which they are common is not the entire human species. It’s only people who decided to look for and do this kind of genetic testing—and it only included people whose genetic profiles indicated their ancestors were European, a group that is already disproportionately well-represented in genetic studies.

Still, scientists who weren't involved in the research thought the findings were valid. "I think it’s a really nice piece of work," said Jeffrey Barrett, a statistical geneticist at the Wellcome Trust Sanger Institute. "The highlighted finding meets the standard of statistical significance. Even so, we’ll need studies bigger again by a factor of ten to really crack the genetics of complex personality traits."

In addition to studying how these genes might actually be influencing delay discounting with experiments in mice, expanding the population is certainly a long-term goal for Palmer and Sanchez-Roige. They hope to get 100,000 people involved in the study by the end of 2018.