This is anecdotal, however I remember hearing about farmers trying to maximize for egg production. Multiple hens would share a cage. The initial criteria was to only breed the hen that produced the most eggs. Overall egg production declined though. It turns out that the best performing hen was doing so at the cost of the others, ie. eating more than its fair share, being combative. The strategy that worked in raising overall egg production was breeding all the hens from the best performing cage.