Lancaster County prosecutors also said they will seek to try 24-year-old Andrew Moyer Sr. and 23-year-old Brianna Michael together in the October death of 6-week-old Andrew Moyer Jr.
The Columbia residents are charged with homicide, conspiracy and endangering the welfare of a child.
Authorities say the infant was abused for at least a week before a final beating caused fatal brain injuries.
Moyer's attorney, Barry Goldman, said he wasn't surprised by the decision. He told The (Lancaster) Intelligencer Journal/Lancaster New Era that he would use a private investigator and medical professionals to probe the mental health status of his client.
Michael's attorney, Merrill Spahn, declined comment.