No comprehensive and single hypothesis has been put forward to account for the phenomena grouped under the term heterophoria. Certain conditions underlying it have been mentioned, such as overdevelopment, faulty insertion or length of the muscle. The so-called declinations described by Stevens have also been considered responsible for some of these phenomena. Aniseikonia, too, has been mentioned as a possible cause, as has paresis or partial paralysis. The chief character of these anomalies is their variability.
These conditions may be said to be underlying anatomic states rather than causes. It is necessary to take a step farther back and ask what is the cause of these underlying states. It seems lacking in reason to attribute a variety of possibly causative conditions to phenomena which are fairly uniform in their manifestation, if one excludes the cases in which the etiologic factor is definitely paralysis or trauma.
When the subject
MARLOW FW. A TENTATIVE INTERPRETATION OF THE FINDINGS OF THE PROLONGED OCCLUSION TEST ON AN EVOLUTIONARY BASIS. Arch Ophthalmol. 1938;19(2):194–204. doi:10.1001/archopht.1938.00850140036004