Interesting, although I cant quite grasp how introducing the law would make juries more likely to convict...
Because if the jurors felt that the person was guilty, but that the punishment was excessive for the crime, they would essentially nullify the verdict.
Back in the early 19th century, you could be hanged for something as trivial as stealing a silk hankie from somebody’s pocket. As time wore on, juries consisted of people who thought that death for comparatively trivial crimes was too much and these petty criminals were being let of in large numbers because they didn’t want their deaths on their conscience.
The same principle was applied as sympathy for the mentally ill and deranged increased during the early 20th century. I myself believe in the death penalty for pre-meditated murder, but I don’t think I would be at all enthusiastic about convicting a mother with post-natal depression who killed her child whilst her mind was unbalanced if the only available crime that she could be convicted of was that of capital murder.