I would argue that intelligence (as is muscle mass for example) is dependent on the need, and hence, training for it. If your life is simple and repetitive, why

I would argue that intelligence (as is muscle mass for example) is dependent on the need, and hence, training for it. If your life is simple and repetitive, why focus on getting smarter as it will probably make you bored more easily. Similarly, if you do physical work all day your muscles are bound to adapt.