The authors present a retrospective case review of acquired esotropia. An internal database was reviewed for cases of esotropia and divergence insufficiency between 1978 and 2018 and seen by a single ophthalmologist. Patients with initial presentation of esotropia onset after 10 years of age were included; any patients with history of strabismus surgery were excluded. With the aim of identifying a homogenous population of divergence insufficiency, acquired esotropia which may mimic divergence insufficiency and esotropia with atypical misalignment patterns were first segregated. The remaining acquired esotropia were split into divergence insufficiency or not, using a greater than five prism dioptre larger distance measurement than near as the criteria. Six hundred and forty-six acquired esotropia were identified from the database, with 256 segregated and analysed as not having divergence insufficiency. The authors report an increasing incidence of divergence insufficiency over the 41-year period studied from 12% in the first two decades and 29% in the latter two decades. This increase is still seen when adjusted for age. Uniform criteria were set by the authors and did not rely on physician coding at the time of assessment. The authors present a new hypothesis for the aetiology of divergence insufficiency linked to chronic development of increased convergence tonus linked to progressive addition lenses and / or prolonged near work. This new theory requires assessment in prospective studies.