A comment by Ran in this thread was so interesting I thought I’d give it its own post. He quotes from Bescherelle: La Conjugaison pour tous, a comprehensive description of French verbal conjugation (I’ll give his translation, slightly emended by me, since the original French is available in his comment):
131 Some remarks on past participle agreement
The subject of past participle agreement involves significant developments that could suggest that it’s one of the most important aspects of the language. To take an accurate measure of the import of the problem, the following remarks should be kept in mind.
– A matter of spelling
Past participle agreement is almost exclusively a matter of spelling. Gender agreement makes itself heard in speech in only a small number of participles: for example, offert. By far the greater number of past participles have masculine forms ending in -é, -i, or -u, and only mark their feminine forms in spelling: -ée, -ie, -ue. As for agreement in number, it never manifests itself in speech, except in the case of liaison, itself rather rare.
– Little-respected rules
Even in those cases where gender agreement is apparent in speech, we often find, in today’s language, that the rules aren’t observed, notably for the agreement of a past participle with a preceding direct object. We very often hear *les règles que nous avons enfreint or *les fautes que nous avons commis instead of the regular enfreintes and commises.
– An artificial rule
The rule of agreement of a past participle with a preceding object is one of the most artificial in the French language. Its introduction can be dated with precision: the poet Clément Marot formulated it in 1538. Marot took as his example Italian, which has since partially abandoned this rule.
– A political matter?
Marot’s rule was nearly abolished politically. In 1900, a courageous minister of public education, Georges Leygues, published an order that “allowed” [tolérait] non-agreement. But the French Academy brought so much pressure to bear that the Minister was forced to replace his order in 1901 with a text that did away with the acceptance of non-agreement except when the participle is followed by an infinitive or a past or present participle: les cochons sauvages que l’on a trouvé or trouvés errant dans les bois.
This little story is a perfect illustration of the idiocy both of imposing artificial rules on a living language and of allowing academies to keep the language from throwing them off. Georges Leygues, je vous salue!