Comment by eMPee584
Low Protein intake => low neurotransmitter levels => lower emotional balance / control is a causal chain I've learned about a few years ago from a therapist named Julia Ross who treated thousands of patients with this insight and published three books about it, the most recent one (The craving cure) being a comprehensive practical resource about this phenomenon. It seems to be a major factor in depression, addiction, obesity, and dysfunctional social behaviour.. and little surprise it affects chicken, too.
In my view, we have massive problems (child brain development, social problems) in the world because of protein scarcity, as capitalism excels providing everyone with ample cheap carbs but cheap sustainable protein, not so much. I dream of open source bioreactors for algae (spirulina etc) too boost availability of Protein & Omega 3 (which is another hugely undersupplied nutrient, esp. in non-coastal regions and as appetite for sunflower-fried batter goes up, because Omega 6 cancels out 3).. here in Dresden, we have a small start-up https://algenwerk.de that is trying to commercialize it but the cost really has to be brought down a lot, rn one jar is about 8€ for some green goo that tastes like nothing, but it has potential and they are a talented team.
I'm no dietitian, but for the "cheap, broadly available protien" I think beans and lentils fit the bill. I do not know if they are sustainable, which you mention as a requirement in your post, but surely plant based is more sustainable than meat.