Most firstborns I've come across exhibit one of two Main characteristics - they're either driven by a heightened sense of responsibility or are very lax. They seem drawn to the extremes of such mutually exclusive behaviours.
The joy of being firstborn for many begins at a very young age and ends when another sibling shows up. From being the center of attention to setting the example for younger ones to follow. It is an often burdensome task that sometimes weighs the most caring of elder siblings down.
This is especially true in developing countries where large families and low incomes mean the elder sometimes provide financial support for several family members.
But it doesn't stop there, it also comes with constant comparisons to your siblings and your parents.
Constantly under pressure, some eventually break and succumb to psychological pressure by drifting totally off course from expectations. For others, making everyone happy usually comes at a price - their happiness.
A closer bond between siblings from birth can promote a sense of shared responsibility and reduce the burden faced by firstborns. Am quite sure some of us can relate.