Should V.S May in subjunctive mood

In response to a similar question on WordReference.com:

He is working hard for fear that he ____ fall behind others.

A should B would?

Thomas Tompion responds

The should/shall in the first person convention is appropriate for the ordinary future and conditional. Here we have a subjunctive created by a modal auxiliary and this would naturally be should rather than would where we wanted to give a more formal feel to the expression. One could also use might.

I agree with [the previous contributor] that one could use either should or would here, or one might elide the auxiliary altogether – which would give an even greater feel of formality.

Here are instances in the GB and US corpuses [via Google ngrams] of the different forms.

• For fear that he should fall behind .... 0 (GB) - 0 (US)
• For fear that he would fall behind ..... 1 (GB) - 14 (US)
• For fear that he might fall behind ..... 2 (GB) - 7 (US)
• For fear that he fall behind ................ 1 (GB) - 0 (US)

Lest anyone think that means that the form with should isn't to be found, here are two examples from literature, one American, the other British.

• And, for fear that he should suspect how her heart was aching, she gave a particularly brilliant and joyous smile - [1911] Eleanor H. Porter » Miss Billy Married » Chapter XX. Arkwright's Eyes are Opened.
• Is there any one you want to see, or anything of that sort?' and doing it as steadily as possible for fear that he should mistake the carelessness of whisky for the distraction of fear, I got my candle alight - [1902] H.G. Wells » The Story of the Inexperienced Ghost.

The Google 5grams illustrate the increasing currency of would against should in this construction, in both American and British English.

The most idiomatic answers would use '... would fall behand' and ' ... might fall behind'.