The eldest child generally has the highest IQ, according to a study from the University of Leipzig - and every successive child in the family has a lower IQ than the last.
This is believed to be because of upbringing, rather than biology. Younger siblings are generally given more freedom to do what they want, whereas there is pressure on the elder sibling to achieve.
Younger children also often think of themselves as less clever than their siblings, although the difference is relatively small, at around 1.5 IQ points for each younger sibling.
The researchers said that the difference could be down to older children getting some time where they have their parents' complete attention before their siblings are born.
They also suggested that passing on knowledge to their younger siblings could play a part in increasing their IQ.
Leipzing University's analysis came from the results of three national studies, including one from the UK, involving more than 20,000 people.
The data included results of IQ and personality tests, and the analysis showed a clear drop in IQ and a stark drop in the person's own perception of their intelligence with each extra sibling.
However, contrary to popular belief, there were no differences in sibling personalities, despite many people thinking that younger siblings are funnier or more extroverted.
Researcher Julie Rohrer, who co-authored the research, said: "One theory is that following children “dilute” the resources of their parents.
"While the firstborn gets full parental attention, at least for some months or years, late-borns will have to share from the beginning.
"Another possible factor is described by the tutoring hypothesis: A firstborn can “tutor” their younger siblings, explaining to them how the world works and so on.
"Teaching other people has high cognitive demands – the children need to recall their own knowledge, structure it and think of a good way to explain it to younger siblings, which could provide a boost to intelligence for some firstborns."
AT A GLANCE
Myths about personality differences in siblings
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