The study, published in Evolution and Human Behavior, was conducted in six countries and presents data on the effect of birth order and parental favoritism on behavioral trends. According to the findings, lastborn children are both the most favored by parents and the most rebellious.

“As mothers begin to reach the end of their reproductive career, it makes good Darwinian sense that they invest heavily in lastborns while they’re still vulnerable to disease, because lastborns are irreplaceable,” said Frank J. Sulloway, a visiting scholar in the Department of Psychology.

Rebellion, which is directed not only towards parents but also the “pseudo-parental authority” of older siblings, is partly an act of “sibling deidentification,” a lastborn’s attempt to disengage from family values. This process of deidentification results in reduced comparisons to older siblings, since firstborns were found to be the most likely to identify with parental values.

Not surprisingly, firstborns were also found to be the closest to their parents, and lastborns the least close. Middle children, by contrast, were more likely to name a sibling as their closest kin, but only when the mother was relatively old at the time of the participant’s birth. Scientists call this a “quadratic” (as opposed to “linear”) effect.

“The middleborn is never alone in the family,” Sulloway said. “When you’re the middle child, you’re getting only a third of everything. The only way for parents to invest equally in their offspring is for them to systematically favor the middle child. That almost never happens.”

As the mother’s age increases, such effects are magnified, as mothers tend to invest more energy into the oldest, who have more future reproductive value, and the youngest, who are generally more needy and irreplaceable. Middle children are often overlooked.

To compensate for this parental investment loss, middleborns become the most peer-oriented and agreeable.

“Middle children need to have a wider ranger of strategies for negotiating, given that they’re sandwiched in the middle,” Sulloway said. “This set of strategies translates into more diplomatic behavior.”

Other siblings also occupy special “family niches.” Firstborns, who tend to adopt surrogate parental roles, have been found to be generally more responsible. Sulloway calls this “firstborn conscientiousness.”

“This tendency shows up in better grades and higher test scores on achievement tests, and a greater propensity to go to upper level colleges,” Sulloway said.

As niches are taken up by older siblings, lastborn children are more creative in finding alternative niches and become more open to experience.

“Younger siblings have been more willing to question the system and, throughout history, to lead radical revolutions,” Sulloway said, whose best-selling book “Born to Rebel,” was devoted to this topic.

Sulloway and his colleagues administered questionnaires to university students throughout the world. Participants had “intact family environments,”—both parents and all full (not half or step) siblings until age 10.

Participants named who they thought were the most favored sibling and the rebel of the family. They were also presented with a scenario involving the witnessing of a car accident, and asked to whom they would turn for emotional support.

The findings aid psychological analysis of personality development and the understanding of family relationships.