They fixate on what is unfair and cannot move past it
ADHD child behaviour
ADHD justice sensitivity turns unfairness into a pain-level response, and the feeling stays stuck in a loop because it has not been discharged. Logic will not release it; only validation does. So validate the feeling before the interpretation. Say that feels really unfair and mean it, then let it land before explaining anything. The acknowledgement is the exit. Once the feeling is seen, your reasoning can finally get through.
What to do right now
The unfairness is consuming them. The sibling got a bigger piece. The rule changed without warning. The teacher picked someone else. ADHD justice sensitivity means unfairness produces a pain-level response. They cannot 'just let it go' because the feeling is stuck in a loop. Validate the feeling, not the interpretation. 'I can see that feels really unfair' is more regulating than explaining why it was actually fair.
What your brain just did
Your body
Your nervous system is activated. Cortisol and adrenaline are influencing how you think and react right now. This is physiology, not a character flaw.
Your brain
The ADHD prefrontal cortex provides less reliable braking between feeling and action. The gap between trigger and response is neurologically shorter than in a neurotypical brain.
What this did
What happened makes sense when you understand how your brain is wired. The reaction isn't the problem to solve. The conditions that led to it are.
What your child is experiencing
Their body
The unfairness is stuck. They know they should move on. They cannot. The loop is producing distress that is now bigger than the original event.
Their brain
ADHD emotional processing loops persist because the feeling has not been adequately discharged. Logic does not discharge emotion. Validation does. The loop will continue until the feeling is acknowledged.
What they need
Validate first, reason later. 'That feels really unfair' has to come before 'but here is why it happened.' The validation is the exit. Once the feeling is acknowledged, the loop can release.