A recent human behavior study co-authored
by a UC Berkeley researcher might provide the reason why
the youngest sibling always gets the biggest scoop of
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
“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
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
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
The findings aid psychological analysis of
personality development and the understanding of family
“Such studies help parents and siblings to understand
that behaviors and relationships they took as personal
are relatively universal,” Sulloway said. “This
realization helps to depersonalize that conflict.”