Your gateway to endless inspiration
Captain Arthur Hastings is so ADHD coded.
Every episode he has a new hyperfixation, that he has abandoned by the next one. His car is pretty much his only constant interest but even then he plans to participate in a 24 HOUR race without having any experience with racing. We see how easily a new hyperfixation is triggered with Hastings in one episode (don't know exactly which one) when he and Poirot visit some attorney who deals with the stock market and not shortly after Hastings is absolutely obsessed with it. It screams ADHD to me.
In 'The Wasps Nest', when Hastings is currently obsessed with photography, Poiroit even says something along the lines of "his newest tick, I'll give him two weeks" (rough translation from the German synchronisation since the English original is not available anywhere in Germany as far as I know ...). So we have canon proof that Hastings getting new obsessions/hobbies and abandoning them shortly after is indeed happening and not just the writers not showing us how he pursues his interest anymore.
I'd also attribute his naivety/slowness (some people say he's stupid but I won't) to ADHD. I myself and most people I know with ADHD keep getting told that we are naive or gullible. Taking people at face value and generally not assuming ill intent is common in people with ADHD and something we also see in Hastings.
His slowness / him not being really smart is a characteristic that is certainly being amplified by the fact that we constantly see him in comparison to Poirot (I doubt that he would be seen as that stupid if the show was just him and Japp). I'd also say that him not being able to follow Poirot's deduction and reasoning doesn't mean that he is necessarily dumb, just that he is not as intelligent as Poirot and he does have his smart moments. (Also I think he was meant to represent the audience, and we often also have no clue what'S going on) But his less bright moments can also be attributed to ADHD. ADHD causes brain frog , as well as concentration and memory issues ( If I don't write important appointments down on my arm I will forget them no matter how important, but I can tell you everything I've learned about vulcanos when I was 8). He gets distracted easily, fails to pay attention to the 'important aspects' of cases and would rather tell everyone about his newest hyperfixation or work on his car, but he's trying his best. In his areas of expertise (cars, or whatever he is currently obsessed with) he is smart and knows his way around, he's just at loss when it comes to everything else.
I forgot halfway through where I was going with this soooo....
Captain Arthur Hastings OBE has ADHD!