Besides the already mentioned Rhesus-monkey experiment, there’s also the “evidence” of the children found in Russian and Romanian orphanages after the fall of the Iron Curtain, who received feeding but no stimulation and had severe lack of human contact, due to the shortages of personnel.
Anecdotally, there’s also the famous experiment of Kaiser Friedrich II where, trying to find the original language of mankind, separated several children from their mothers; the nursing mothers were not allowed to speak with them or touch them, but instead of talking, the babies died soon.
With regards to feral children, who are almost always (known cases, separate from legends) emotionally and developmentally retarded, scholars are still unclear whether the children are retarded because they were abandoned, or if they were abandoned because they showed signs of retardation; obviously this is impossible to find out later, but it is clear that abandoning children is not good for them.