Much research has been done on Adler's birth theory and middle children since the 1960s, but much of it is contradicting itself.
Given why middle children are what they are, a comprehensive book by two therapists entitled The Middle Generation Syndrome states, "If there are only three children, the first becomes the oldest and the youngest becomes the baby. That middle child can be omitted […] The closer the children are in old age, the less energy the parents had to give, which exacerbates the problem. "
And there is some research that suggests the order of birth could affect personality and mental health: for example, a 1988 publication in the Journal of Genetic Psychology found that firstborn children were less likely to be depressed after analyzing 404 children and anxiety suffered as middle and younger children. They also tended to report higher self-esteem.
But for every study that finds birth order traits to be legitimate, there is one that concludes that it isn't.
A 2015 article titled "Investigating the Effects of Birth Order on Personality" states: "We consistently found no effects of birth order on extraversion, emotional stability, tolerance, conscientiousness or imagination […] lasting effect broad personality traits outside the intellectual realm. "
In response to this research, another study, boldly titled "Settling the Debate on Birth Order and Personality" concludes that "Birth order has little or no significant relationship with the development of personality traits and a tiny relationship with the development of Has intelligence ".
So mean child syndrome is unlikely to be a real medical diagnosis in the near future. However, this does not mean that middle children cannot relate to or benefit from paying attention to the general characteristics of "middle child syndrome". Here's some background on what they are and how you can use them to your advantage.