It's my guess that it is rare that someone decides the tilt of their head. Posture is cultural, it varies from culture to culture. In industrialized nations most people have excessively curved spines and as a result most heads tilt back, as shown in the first photo. When people have balanced posture the face is vertical, as in the second photo. The study gives another reason (an appearance of cooperation) to balance your posture, so your bones are on the axial line of gravity.