Few paintings are as instantly recognisable as Grant Wood's American Gothic, and few scholars have spent as long asking why. Wanda Corn, an eminent historian of American art, has published five decades of research in The Couple with the Pitchfork: American Gothic from Easel to Icon, tracing the 1930 picture from regional entry to global symbol. A Midwest descendant herself, she includes a photograph of her grandparents outside their Indiana home, an echo of Wood's composition.

Raised in Iowa, Wood sought a way of working distinctive to the Midwest after trying French Impressionism in Paris and decorating in Cedar Rapids; small-town arts funding, including Carnegie Foundation support, opened room for a regional voice. He modelled the figures on his sister Nan and his dentist, intending a father and daughter, but audiences read a married couple, the press a farmer and wife. Iowans called the pitchfork outdated and the woman dour; critics found the work out of step with abstraction.

Corn reads the picture as pattern and form, not plain realism: verticals from pitchfork tines to church steeple order it, while round and oval shapes, long faces, brooch, calico pattern and the man's ears, harmonise it. Wood prized a decorative quality of hard geometries, painted so precisely the work seemed self-evident rather than deeply thought. Those clear forms reproduced well in newsprint, and after a prize at the Art Institute of Chicago the painting became a sensation.

For Corn the misunderstanding is the key, not an error. All agreed it showed marriage, an archetype from Egyptian princes to Dutch portraits, endlessly adaptable: her archive holds hundreds of reworkings, from state-fair butter sculptures to versions backing same-sex marriage or sending the woman to work. Early adopters included Black Chicago artists of the 1930s, who used the couple-before-a-house motif for migration stories, as Walter Ellison did. Beneath the smooth surface Corn finds unease, sharpened by Wood's failed marriage five years later.