Since the 19th century, scientists have debated the possibility that birth order may influence individual qualities as profound as personality and intelligence, with some recent evidence on IQ suggesting younger siblings may truly be the least of our brethren. Now, a sociologist at Stockholm University says the youngest in a family may also be the most likely to die by suicide.

"Our findings are important, since they highlight that birth order should be considered an early-life circumstance that determines mental health across the life course," study leader Mikael Rostila, told LiveScience.

Significantly, the researchers found an 18 percent increase in suicides for every step down in the sibling hierarchy. In a paper published this month in the American Journal of Epidemiology, they analyzed data collected by the Swedish government on sibling groups born from 1932 to 1980, who were later observed between 1981 and 2002. For whatever reason, the association between lower birth order and higher suicidality rose from those born between 1932 to 1955, and those born later, between 1967 and 1980.

“Further analyses suggested that the association between birth order and suicide was only modestly influenced by sex, birth spacing, size of the sibling group, own socioeconomic position, own marital status, and socioeconomic rank within the sibling group,” the researchers wrote. “Causes of death other than suicide and other external causes were not associated with birth order.”

Similarly, a 2009 study from Germany found a link between birth order and suicidality, though with slightly different findings. In a paper published in Psychological Health Medicine, investigators sampled more than 2,500 children and adolescents seen in a psychiatry clinic for self-injurious behavior as well as other behavioral problems. In that study, middle children were found to be the most likely to exhibit such behavior, the researchers concluded.

Suicide is one of the leading causes of death for younger people around the world, killing a million people every year. The Centers for Disease Control and Prevention says more than 38,000 Americans committed suicide in 2010, as the 10th leading cause of death. Every 13.7 minutes, another American takes his or her own life.

Source: Rostila, Mikael, Saarela, Jan, Kawachi, Ichiro. Birth Order and Suicide in Adulthood: Evidence From Swedish Population Data. American Journal of Epidemiological. 2014.