- Lack of smiles or other joyful expressions by 6 months
- Lack of response to name by 7 months
- Lack of back-and-forth sharing of sounds, smiles, or other facial expressions by 9 months
- No babbling by 12 months
- Does not point, reach, wave, or make back-and-forth gestures by 12 months
- No words by 16 months
- No two-word sentences by 24 months
- Lack of coordination of nonverbal communication
- Little variation in pitch or intonation, irregular rhythm, or unusual voice quality
- Loss of speech or babbling at any age