The first one is more intelligent with a higher IQ than the later siblings probably due to the fact that he or she is the 'pearl of the eye' and gets lavish attention from parents that boosts the intellectual development.
But medical literature antagonizes this.
Recently, evidence has emerged on the consequences of primogeniture on long-term metabolic and cardiovascular health risks.
There has been a steady decline in the birth rates in the world owing to many factors which include family planning measures, government policies (one child norm) or economic constraints. As a result of this, we have a large percentage of one-child families. Since the first child is prone to health issues, we are now staring at a future population with a high proportion of risk factors.
Despite similar gestational ages, first borns are 250g lighter than the later siblings at birth. Changes in the placentation are accounted for this difference in birth weights.
Low birth weight is an indirect indicator of poor intrauterine milieu and adverse health outcomes have been demonstrated in association with decreasing birth weight. Offspring of mothers exposed to nutritional or physiological stress during pregnancy are at increased risk of metabolic and cardiovascular disease in later life. There is extensive evidence linking a reduction in birth weight with increased risk of insulin resistance, Type 2 diabetes mellitus, hypertension, cardiovascular disease, stroke and cancer.
First-born children are taller than the second or third child. Taller stature in childhood is positively associated with overweight status and obesity later in life.
Various statistics suggest that first-borns were more likely to develop Type 1 diabetes mellitus, hypertension, heart disease, allergic disorders and psychological issues.
Primogeniture are the largest identifiable study group (accounting for more than half of the world's population), so that any associated health risks could have enormous public health consequences.