The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 6, 'Capitals'
6.3 Movements and Periods
Capitals must be used for nouns and adjectives denoting cultural, philosophical, literary, critical, and artistic movements and periods when these are derived from proper nouns:
Cartesian, Chomskyan, Christian, Erastian, Freudian, Platonism
They should also be used for literary and other movements when the use of a lower-case initial might cause confusion with the same word in a more general sense:
a poet of the Romantic school
a novel with a straightforwardly romantic plot
This covers the use of capitals when terms such as Conservative, Democrat(ic), Independent, Liberal, National(ist), Republican, Social(ist) refer to specific political parties or movements, e.g. the Independent Labour Party, the Social and Liberal Democrats, but not otherwise, e.g. ‘a man of conservative (or liberal) views’.
For movements and periods with the prefix ‘neo’ and other compounds, see 6.5.