The things that point towards sexual selection being involved are rapid growth, extreme growth, any sexual dimorphism in the organ involved, and its costliness.
The alternative to sexual selection is natural selection—the idea that big brains helped our ancestors to survive. This seems less plausible as a driver of a large brain—since mere survival is quite a bit easier than survival and successfully raising numerous babies.
The things that point towards sexual selection being involved are rapid growth, extreme growth, any sexual dimorphism in the organ involved, and its costliness.
The alternative to sexual selection is natural selection—the idea that big brains helped our ancestors to survive. This seems less plausible as a driver of a large brain—since mere survival is quite a bit easier than survival and successfully raising numerous babies.