Do you have an older sibling who always insists he or she is smarter than you are? Your fears may not be unfounded: they really might be.

A study of more than 240,000 Norwegian men found that older siblings score higher on IQ tests than their younger brothers and sisters. In cases where the first child dies in infancy, however, the second-born child raised as the firstborn assumes the mantle, performing as well as the actual elder child on intelligence exams.

Study co-authors Petter Kristensen, a professor of epidemiology and occupational medicine at the University of Oslo, and Tor Bjerkedal, an epidemiologist with the Norwegian Armed Forces Medical Services, reviewed a data set on 241,310 men, all 18 or 19 years of age, who were given IQ tests after being drafted into the army from the mid-1960s to mid-1970s. This gave the researchers access to an abundance of data on the test takers as well as, in many instances, on other children within their families.

Their findings, reported today in Science: on average, firstborn males had an IQ of roughly 103.2, whereas the second-born child scored about 100.4 and third-borns 99. When the duo accounted for social rank, however, it turned out actual birth order may not be the key to intelligence.

"We made the assumption that the actual rank among children in a family is the social rank—assuming that a second-born in a family where the first child was deceased would be social rank one and biological rank two," Kristensen says.

After this adjustment, the scores of second-born children raised as the eldest kid in their family jumped to 103 and the average IQs of third-borns rose to 100.3. Third-born children who had lost two elder brothers averaged the highest IQs of all, scoring 103.5.

Kristensen says that his findings indicate that this effect is likely explained through simple resource allocation: "When there are more children, he notes, the resources will be more scarce for everyone compared with the firstborn who gets all the attention with no competition."

"Critics have long argued that such birth-order effects, which typically emerge in between-family studies, are spurious—phantom artifacts of uncontrolled differences in family size, socioeconomic status, parental IQ and other background factors," wrote Frank Sulloway, a visiting scholar at the University of California, Berkeley's Institute of Personality and Social Research, in an editorial accompanying the study. "At least in the domain of intellectual ability, the new Norwegian findings rule out this alternative explanation."