Time magazine has a great article discussing psychological differences that have been picked up by research looking at birth order effects. Interestingly, while first and last borns seems to have distinct traits, middle children are still a bit of a mystery.
Birth order effects seem to be one of those things that can be reliably found when examining large groups but, because of the large amount of individual variation, strong effects are not reliably present on the level of single families.
Nevertheless, the research has found over the population there are, on average, some interesting psychological differences linked to birth order – particularly between first and last borns.
…personality tests show that while firstborns score especially well on the dimension of temperament known as conscientiousness ‚Äî a sense of general responsibility and follow-through ‚Äî later-borns score higher on what’s known as agreeableness, or the simple ability to get along in the world. “Kids recognize a good low-power strategy,” says Sulloway. “It’s the way any sensible organism sizes up the niches that are available.”
Even more impressive is how early younger siblings develop what’s known as the theory of mind. Very small children have a hard time distinguishing the things they know from the things they assume other people know. A toddler who watches an adult hide a toy will expect that anyone who walks into the room afterward will also know where to find it, reckoning that all knowledge is universal knowledge. It usually takes a child until age 3 to learn that that’s not so. For children who have at least one elder sibling, however, the realization typically comes earlier. “When you’re less powerful, it’s advantageous to be able to anticipate what’s going on in someone else’s mind,” says Sulloway.
We featured some studies previously on Mind Hacks that suggested that first born children have marginally higher IQ scores, although a similar study in Thai medical students found the reverse effect, younger siblings tended to be more intelligent.
This highlights the role of culture in these effects, and the Time article illustrates a similar point with regards to girls. Perhaps fifty years ago when girls were less expected to go to college and have careers, the birth order effect may have been much less clear because of the cultural limitations on female work and education.
Now the cultural expectations have changed, the effect of birth order on psychological development may also be different.
Link to Time article ‘The Power of Birth Order’.