The Mid-Point on a Rating Scale: Is it Desirable?
http://marketing-bulletin.massey.ac.nz/V2/MB_V2_N3_Garland.pdf
  peer-reviewed

Garland, Ron
Marketing Bulletin
1991

Abstract:

This study examined the effect on survey results of having no neutral or mid-point on a Likert scale. Participants in a face-to-face omnibus survey were shown either a five point (with mid -point) or four point (no mid-point) Likert scale of importance on a card and asked to state their opinion about the importance of product labelling (additives, ingredients etc.) on packaged foods. This research provides some evidence that social desirability bias, arising from respondents' desires to please the interviewer or appear helpful or not be seen to give what they perceive to be a socially unacceptable answer, can be minimised by eliminating the mid-point ('neither... nor', uncertain etc.) category from Likert scales. There is also some evidence that the presence or absence of a mid-point on an importance scale produces distortions in the results obtained.