The Little Girl Voice is an informal name for a vocal trait in adult women that is caused by psychological trauma before the onset of puberty. Women that are affected speak in a higher sounding, child-like pattern, usually in a manner similar to the age at which they suffered the traumatic event.
Dr. Drew frequently uses this as a cue when talking to female callers, in order to better understand the cause of their problems. He is so practiced at noticing it that he can often guess down to a 6 month period in the woman's life when she was traumatized, and sometimes even what the trauma was, much to everyone's amazement. Sometimes callers lie or obfuscate about having a trauma history at first, claiming their childhood was completely normal, only to reveal something horrific later in the conversation that precisely fits with Drew's initial deduction.
Dr. Drew discovered this phenomenon after practicing medicine for many years and noticing a very distinct pattern with his patients, whom he often converses with personally during the treatment process. He noticed all the female patients who talked a certain way had a trauma history (usually sexual abuse) that they would reveal during the session. Strangely, there has been no formal scientific study of this phenomenon, though its reality is undeniable in the face of call after call that the doctor correctly deduces, sometimes with them only saying a single word. Drew surmises that the incident of trauma "freezes" some portion of the brain's development, resulting in the speech pattern remaining the same into adulthood. He also notes that women who undergo long and intensive treatment usually lose this pattern and speak normally.
Despite its lack of formal research, many others in the mental health field to have noticed this pattern too, though detecting it correctly requires practice. Laypeople with sufficient experience can learn to pick up on it, though they can sometimes be mistaken if person merely has a high-pitched voice. Drew often suggests closing one's eyes and imagining that you know nothing about the speaker (including their real age) and concentrating on how old the speaker makes you feel like they are.