Over the past decade, rising national wealth across high-income countries has contributed to some improvements in health and well-being among adolescents. But the gap in health between rich and poor has widened, an international study of nearly half a million adolescents from 34 countries  across Europe and North America has found.
The findings, published in The Lancet, reveal that socioeconomic differences across multiple areas of adolescent mental and physical health increased between 2002 and 2010, with young people from the poorest socioeconomic groups more likely to be in worse health: being less physically active, with larger body mass index (BMI), and reporting more physical and psychological symptoms (such as irritability or headaches).
"A strong international focus on reducing child poverty and mortality in children under 5 years has not been matched by a similar response in older age groups, resulting in widening socioeconomic inequalities in adolescent health,"  explains Frank Elgar, lead author and a psychiatry Professor at McGill University in Quebec, Canada.
"If health inequalities are now widening in such abundantly rich countries, particularly during the so-called 'healthy years' of adolescence, then these trends are especially alarming for future population health." 
The aim of the study was to measure socioeconomic-related inequalities in five areas of adolescent health (physical activity, body mass index, psychological symptoms [irritability, feeling low, feeling nervous, and difficulty sleeping] and physical symptoms [headache, stomach ache, backache, and feeling dizzy, and life satisfaction]), and to track their changes between 2002 and 2010.
The researchers analysed nationally representative data from almost 500000 young people across Europe and North America who participated in the WHO Health Behaviour in School-aged Children study. The adolescents (age 11 to 15) were surveyed in 2002, 2006, and 2010. Socioeconomic status was based on material assets and common indicators of wealth such a owning a car. The researchers also examined whether differences in health and health inequalities between socioeconomic groups related to national wealth and income inequality.
Between 2002 and 2010, both improvements in health (ie, increased physical activity and life satisfaction) and declines in health (increased body mass and physical symptoms) in young people were recorded. At the same time, the difference in health between the least and the most well-off became larger. For example, the difference in amount of physical activity between the least and most affluent groups increased from 0.79 days to 0.83 days per week. Increasing differences were also found for body mass index (0.15 to 0.18), psychological symptoms (0.58 to 0.67), and physical symptoms (0.21 to 0.26). Only in life satisfaction did inequality decline, from a 0.98 point difference in 2002 to a 0.95 point difference in 2010 (see figure 1 page 3 and table 4 page 5).
The research also showed that adolescents living in countries with greater income inequality were less physically active had larger body mass index, lower life satisfaction, and reported more psychological and physical symptoms. Young people in these countries also had larger health inequalities between socioeconomic groups in psychological and physical symptoms and life satisfaction.
According to Professor Elgar, "The many health and social problems that relate to income inequality and the current global trends in rising income inequality all lead to a grim prediction about future population health. Urgent action is needed to tackle inequities in health in adolescence."
Writing in a linked Comment, John Santelli, Wendy Baldwin, and Jennifer Heitel from Columbia University Mailman School of Public Health, New York, USA point out, "Investment in youth now could pay huge dividends in health outcomes and reduce health disparities in later years...Although some interventions should always support individual behaviour change, Elgar and colleagues remind us of the importance of social context. To improve health and reduce health disparities across the lifespan, a focus should be on social factors that affect the health and wellbeing of young people ."
Notes to Editors:
This study was funded by the Canadian Institutes of Health Research, the Social Sciences and Humanities Research Council of Canada, and the Canada Research Chairs programme. The Health Behaviour in School-aged Children study was funded by public sources in each member country.
 Austria, Belgium (French region), Belgium (Flanders region), Canada, Croatia, Czech Republic, Denmark, England, Estonia, Finland, France, Germany, Greece, Greenland, Hungary, Ireland, Israel, Italy, Latvia, Lithuania, Macedonia, Netherlands, Norway, Poland, Portugal, Russia, Scotland, Slovenia, Spain, Sweden, Switzerland, Ukraine, United States, and Wales.
 Quotes direct from author and cannot be found in text of Article.
 Eg, wealth, access to education, and income inequality