The differences are so less between first-borns and "later-borns" that they are considered to have no practical relevance to personality or intelligence development (IQ) of people, says a study.
The study observed 377,000 high school students and found that first-borns do have higher IQs and consistently different personality traits than those born later in the family chronology.
"This is a conspicuously large sample size," said lead author Brent Roberts in the department of psychology from the University of Illinois.
"It's the biggest in history looking at birth order and personality," said Roberts in the Journal of Research in Personality
"The first-borns enjoy a one-IQ-point advantage over later-borns," said Rodica Damian, a professor of psychology at the University of Houston.
Though the difference is statistically significant, it is practically meaningless. The analysis also revealed consistent differences in personality traits between first-borns and later-borns. The first-borns tended to be more extroverted, agreeable and conscientious, and had less anxiety than later-borns.
"But these differences were small, amounting to a correlation of 0.02," said Roberts.
Previous studies of birth order suffered from small sample sizes as many compared children with their siblings -- a "within-family" design that some assert is better than comparing children from different families.
"The message of this study is that birth order probably should not influence your parenting, because it's not meaningfully related to your kid's personality or IQ," said Damian.