In the first theory, family size plays a role. Parents want to devote the same amount of time and resources to each child. However, when there is more than one child, this time and resources need to be shared. Even with two children, family interest decreases by half. The smaller the family, the more attention, time and resources the child has. As the family grows, these data per child decrease.
The second theory suggests that firstborn children have more opportunities to learn life lessons.
The third theory is the merger theory. If the home environment is an intellectual culture, children will grow up smarter. If you treat a child like an adult, his maturity will increase. As parents become more experienced in raising children, they will be less likely to treat their children like mini-adults. Younger children grow up in a more diluted intellectual environment than their siblings born before them.
The fourth theory is that every child wants attention. While one child wants to be an actor, the other may want to be an athlete. The privilege of being firstborn may lie in the fact that the oldest child can choose the future role that best suits him/her before the competitive environment arises. Oldest children seem to tend to choose intellectual achievements. Researchers from the University of Essex in England found that firstborn children were 16 percent more likely to achieve academically than their younger siblings. This also applies to girls. Eldest daughters are 4 percent more likely to go on to higher education than families with a male first-born.