Wikipedia defines middle child syndrome as,
'the feeling of exclusion by middle children. This effect occurs because the first child is more prone to receiving privileges and responsibilities (by virtue of being the oldest), while the youngest in the family is more likely to receive indulgences.'
I have three children and have always been very conscious of my middle child and preventing her from experiencing this feeling. Luckily for us (and her) she is my only girl and so definitely receives a substantial amount of attention because of this. What I've noticed lately as the youngest gets older, is that the boys tend to gravitate towards each other more and so this is where now I feel the feelings of exclusion may come into play.
Have you experienced this in your house and what have you done to combat it?