I don't have numbers like this well-researched response, but apparently neglected children have higher odds of being diagnosed with ADHD, which is seen through the very high occurrence of ADHD and similar in children who entered foster care at very young ages (in this case, younger than 5 years, well before ADHD is diagnosed). This elevated risk of ADHD apparently doesn't show itself in children put in foster care at later ages.

The specific symptom they were looking at in what I was reading, was how people with ADHD easily walk away from things (friends, family, etc.), never to return. Basically a lot of kids attempt to run away, but people with ADHD are far less likely to return if they succeed in "getting away".

The article was relating having lived the earliest part of their lives being pushed away, to the ability to more easily separate and push everyone away permanently later in life (which they referred to as "shutting down"), and the possibility that ADHD itself might be a coping and survival mechanism.