Hyphens: Beware the Mother of All Plurals

Rasha says:

I am doing research for a school project about family life in Asia, and I came across this headline in The Indian Express:

Mother in laws behind Indian divorces in Malaysia

I know there's a mistake in there somewhere, but I can't put my finger on it.

There are actually two mistakes in that headline: (1) Mother-in-law should always be hyphenated, and (2) the plural is mothers-in-law.

Quick rule:

• To pluralize hyphenated compound nouns, add 's' or 'es' to the base noun. The playoff game was between the two runners-up. The President met with his chiefs-of-staff. The courthouse was filled with attorneys-at-law. 
• If there is no base noun, pluralize the last word. The agents acted as go-betweens during the war.

Correction: Mothers-in-law behind Indian divorces in Malaysia