Firstborns are more intelligent than their younger siblings—at least that's what science tells us. But why birth order matters goes deeper than just getting the best batch of genes from the parental pool. If siblings basically contain the same genetic makeup, how are firstborns different from their younger counterparts? The reasons why those first in line are graced with a higher IQ comes from a cocktail of environmental and biological factors that differ from family to family. How smart its members are depends on everything from early intellectual stimulation to limited parental resources.
While the eldest typically ends up earning higher wages and better education than the rest of the family, being the oldest sibling also comes with a few downsides. Older children are likely to be punished more strictly than their younger siblings. The oldest is also often more of a people-pleaser. These facts about eldest children are pretty surprising—unless of course you're the genius oldest child, then you probably already know all of this.
Being the only child in the house for some time before another sibling comes along gives the eldest an intellectual edge from a very early age. At least for a while, this child will have access to 100% of their parents' resources and attention, whereas a second child can never have the same benefit. Because resources within a family are spread thinner as more children are born, the older child will generally have access before and at a greater level than their siblings. Parents can offer higher level linguistics and usually more educational assistance that what is given to younger children when they don't have to divide their time.
Having more brain stimulation and a higher level of intelligence allows the oldest child to pursue a greater number of opportunities. Their learning capabilities paired with their thirst for approval play a key role in the eldest's drive to seek higher education. Firstborn children are an entire 16% more likely to attend university than any other child in the family. It could also be the reason why eldest children are more likely to snag higher-paying wages.
As early as a year old, the eldest child typically scores higher on IQ tests than younger siblings. In a study tracking the mental growth of kids from the womb until the age of 14, scientists found that the first kid simply gets more attention and intellectual stimulation.
Engaging in educational activities gives the eldest child an advantage in linguistics, cognitive recognition, and complex problem solving. The intelligence gap they express only grows with age. Gaining an intellectual advantage early in life accelerates their growth in contrast to younger siblings who experience less brain stimulation during development and often have a slower learning rate.
The eldest sibling in the family spends more time communicating with adults than with children, which boosts their linguistic skills and their capacity for comprehension. Their younger siblings, however, will spend more time interacting with their elder sibling. While the elder sibling is knowledgeable about complex language, they're typically still at the reading and speaking level of a child. The laterborn is exposed to a less mature environment, and therefore does not gain the same linguistic skills that the older child does.