A cikin littafinsa Dama-Hand, Hagu-Hand, Chris McManus na Jami'ar Kwalejin London ya yi jayayya cewa yawan hagu yana ƙaruwa kuma hagu-hannun mutane a matsayin rukuni sun samar da tarihin sama da matsakaicin adadin manyan masu cin nasara. Ya ce kwakwalwar hagu suna da tsari daban-daban (ta hanyar da ke ƙara yawan damar su) kuma kwayoyin halittar da ke ƙayyade hagu-hannu kuma suna sarrafa ci gaban cibiyoyin harshe na kwakwalwa.Rubuta a cikin Scientific American, McManus ya bayyana cewa, Tambaya: Menene Chris McManus yayi jayayya game da mutanen hagu?
yawan masu amfani da hannun hagu yana ƙaruwa