Whether you’re an only child or the oldest sibling, there are personality traits that are stereotypical to your birth order.
Children without any siblings are typically said to be spoiled and constantly crave attention, whereas firstborn children are found to perform well in leadership positions in their adult life.
Starting with Alfred Adler, research on the correlation between birth order and personality types has been carried out for over 30 years and despite there being common traits across various studies; some psychologists still argue that the order of birth doesn’t actually affect our development at all.
One of the main theories as to why birth order matters is that the children must adopt different techniques and roles in order to win their parents affection, due to the divided attention amongst multiple children.
Parenting expert Michael Grose explains it best, “We’re in a Darwinian struggle from the moment we’re born, fighting for scarce resources within a family – our parents’ time, love and affection.” 
So what has the research actually determined?
The oldest child is usually said to be achievement-oriented and eager to be first in all situations. A 2012 paper  summarising recent studies claimed that first-borns were most likely to gain and hold leadership roles over their sibling counterparts.
First-borns are also more likely to experience a lot of attention over their “firsts”, such as first words, first steps, etc as they’re the first time the parent is experiencing these milestones, causing them to want to seek approval in their later years.
Oldest siblings are overachievers, so it isn’t surprising that the legal profession is swarming with first-borns, along with the fact that a lot of world leaders are first-born children. It does have a negative side, however, with first-borns being prone to jealousy, anxiety and defensiveness since they’re the only sibling to experience a parent’s full undivided attention and then have to share it later on in their childhood.
According to Darwinian theory, the middle child is neither the precious first or the vulnerable youngest, so lucks out when it comes to the birth order lottery. The middle child is often noted to be the peace keeper who makes compromises and negotiates as is stuck between the other two.
Due to this, they’re often good at connecting with people both older and younger than them and are said to be friendlier, more faithful and loyal. Being in the middle, the child is said to be more relaxed without the pressure of being the first or last child – also meaning they may lack competitive drive.
Finally, the youngest child’s personality is said to be vastly different from their other siblings. Due to the changing nature of parenthood from the oldest to the youngest child, the youngest is said to be more rebellious, attention-seeking and creative by nature.
As the parents become more comfortable and relaxed, having been through the process already, the youngest child may feel as though they don’t get the attention they deserve and perhaps that they perceive the older child receives.
Therefore, the youngest child may adopt a social and outgoing persona in order to gain the attention they need, possibly even resorting to manipulation to get what they want.
Whilst birth order does seem to have an impact on the personality of siblings, other factors must be considered such as temperament, gender, age gap, genetics, parenting and environment.
There are comparisons to be made amongst different-age-same-gender siblings such as more competition than different-gender siblings and whilst these studies help us gain understanding into the birth order personality types, they’re not a fixed state and as we grow into our adult lives, we may shift away from the people we were as children.