In work that gives cranky teenagers another reason to blame their parents for all life's woes, researchers have uncovered a genetic link to happiness.
The study of more than 2,500 Americans revealed two variants of a gene that influenced how satisfied – or dissatisfied – people were with their lot.
Those born with two long versions of the gene (one is passed down from each parent) were more likely to declare themselves "very satisfied" with life than those who inherited two short versions.
The study marks a tentative step towards explaining the mystery of why some people seem naturally happier than others.
"This gives us more insight into the biological mechanisms that influence life satisfaction," said Jan-Emmanuel De Neve, a researcher at the London School of Economics and Political Science.
"If you're feeling down, you can say it's your biology telling you life is less rosy that it is," he added.
A greater understanding of happiness genes might in future allow would-be parents to create a child who will be more satisfied with their life.
Happiness is only partly influenced by genetic makeup. Studies in twins suggest that genes account for roughly a third to a half of the variation in happiness between people. It is not yet known how many genes affect how cheerful we are.
De Neve looked at the genetic makeup of 2,574 people selected to be representative of the general population, whose medical histories were recorded for the US National Longitudinal Study of Adolescent Health. Among the records were answers to a question participants were asked in their early 20s about life satisfaction.
In response to the question, "How satisfied are you with your life as a whole", they answered either "very satisfied", "satisfied", "neither satisfied or dissatisfied", "dissatisfied" or "very dissatisfied".
Writing in the Journal of Human Genetics, De Neve describes how roughly 40% said they were "very satisfied" with life, and among these, 35.4% had two long variants of the gene and only 19.1% had two short versions. Of those who were "dissatisfied" with life, 26.2% had two long variants of the gene, while 20% had two short versions. That indicates a slight over-representation of the long variants in happier people.
The gene, known as 5-HTT, is involved with the transport of serotonin, a feelgood chemical, in the brain. The longer variant leads to more efficient release and recycling of the neurotransmitter.
De Neve calculated that, everything else being equal, having one long version of the gene increased the number of people claiming to be "very satisfied" with life by around 8.5%. Having two long versions raised the number by 17.3%.
In unpublished work, De Neve and other researchers have since replicated the result in a separate group of people.
De Neve urged caution over the result, however, and emphasised that inheriting two short versions of the gene did not condemn a person to a life of misery any more than two long versions would make someone impervious to sadness.
"This gene has an important influence, but you cannot say it causes happiness. Happiness is hugely complex and your experiences throughout the course of your life will remain the dominant force on that," he said.
A 2009 study by Elaine Fox at the University of Essex suggested that people who carried long versions of the 5-HTT gene had a greater tendency to focus on the positives in life. The "bright side" version of the gene might bolster people's resilience to stressful events, and protect against anxiety, depression and other mental health problems.
Ed Diener, a psychologist at the University of Illinois at Urbana-Champaign and author of the 2008 book, Happiness: Unlocking the Mysteries of Psychological Wealth, said: "We are just beginning to understand the actual genetics of happiness, and how genes might influence brain hormones and other physiology that influence our well-being.
"This exciting work offers insights that one day may help us counter disorders such as depression. Parents one day might have the choice of whether to choose genes that will create a child who is more satisfied with his or her life."
Thanks to Ian Sample / Guardian UK
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