The title of this article might trigger self-satisfied smiles among first-borns, and some concerns among the rest of us.
Many studies show children born earlier in the family enjoy better wages and more education, but until now we didn't really know why.
Our recently published findings are the first to suggest advantages of first born siblings start very early in life - around zero to three years old! We observe parents changing their behaviour as new children are born, and offering less cognitive stimulation to children of higher birth order.
The birth order puzzle
It now seems clear that for those born and raised in high-income countries such as the United States, the UK and Norway, earlier-born children enjoy higher wages and education as adults - known as the "birth order effect". Comparing two siblings, the greater the difference in their birth order, the greater the relative benefit to the older child.
However, to date we've had no evidence that explains where such differences come from.
We know it's not an effect of family size, because the effect remains when comparing siblings within the same family and families with the same number of children.
While it makes sense that parents earn more money and gain experience as they get older and have more children, they also need to divide their economic resources and attention among any children that arrive after the first born. We wondered where in childhood these differences began, and what the cause or causes might be.
The origins of the birth order effect
We investigated when birth order differences appear and how they evolve from birth to adolescence. The study involved a longitudinal analysis of around 5,000 American children.
Our findings suggest that birth order differences can start before the age of three. We see an effect of birth order on measures of the physical and social development of children. Such differences increase slightly with age, and show up in a wide array of test scores that measure verbal, reading, math and comprehension abilities.
Somewhat surprisingly, in both our study and in previous ones, there is no evidence that younger children are born disadvantaged: if anything, later-borns are actually on average heavier and healthier at birth. Thus, the birth order effect does not seem to be related to an obvious biological advantage at birth.
Quality of parental investment is key
We explored changes in parental behaviour as a potential contributor to the birth order effect. Our assessment tool was the Home Observation Measurement of the Environment, which provides a measure of the quality of the cognitive stimulation and emotional support provided by a child's family.
We found that children of higher order of birth - that is, those born second, third or further on from the first child - receive less quality parental cognitive stimulation. Our measures encompassed beneficial inputs for the child's cognitive development, such as reading with the child, cultural outings, or availability of musical instruments in the house. They seem to make a difference.
Furthermore, this shift in parental behaviour appears to start in the womb. In pregnancies subsequent to their first, we found that mothers are less likely to reduce drinking and smoking or seek timely prenatal care. Once born, non-first-born babies are breastfed less often.
Birth order does not shape your temperament
Contrary to popular belief, we did not find that birth order is associated with differences in temperament, attachment or behavioural problems among siblings. Regardless of birth position, we also found children to have the same overall self-confidence as teenagers.
Also, we did not see any evidence that parents make any distinction in the emotional support provided to each of their children. Parental interaction aimed at ensuring appropriate emotional development does not diminish for younger siblings.
Relax, you know what you're doing!
Taken together, our findings suggest that a plausible explanation for the negative relation between birth order and educational achievement is a broad shift in parenting, especially with respect to parents' ability to foster early cognitive development.
For most parents, it is probably not difficult to understand how and why parenting focus and behaviour changes with later-born children. Lessons from past experience and additional constraints on time, resources, and attention necessitate adjustments in attitudes and beliefs about what may be possible to accomplish as parents. Parents may choose to relax some non-essential rearing needs for their later born children.
These changes in parental behaviour appear to set later born children on a lower path for cognitive development and academic achievement, with lasting impact on adult economic outcomes.
But it's not all bad news for the younger siblings out there. Regardless of where a child is positioned in terms of birth order, parents support emotional development in equal measure.