Excruciating. Gruelling. Difficult. When dashing A-lister Will Smith opened up about marriage to his beautiful wife, Jada, it was hard to know whether to raise a ragged cheer, or recoil in appalled disbelief, at his unflinching honesty. Because after 20 years together, you can guess how that one played out. While he may have hoped that his domestic disclosure would see him cast as an emotionally courageous hero, during a social media age when everyone is obsessed with being perfect, it is unlikely that Jada will feel that way.

Instead, you can imagine his incandescent missus tearing a strip off him for his big reveal showing her - them, the family - up. The secret of a happy marriage is, as the term suggests, a secret. Just as no mother would dream of telling a pregnant woman the shocking minutiae of childbirth, so do most long-married couples subscribe to an unspoken policy of omerta. Lovebirds and newlyweds look away now, for these are the other truths about marriage that only the bravest among us would dare to reveal.

1 As marriage progresses, each partner will unlearn skills and regress; typically, men who once survived quite successfully in the wild no longer have any idea how to book cinema tickets or buy vegetables. Previously independent women are curiously flummoxed by radio alarm clocks and petrol stations. Infuriating.

2 Shared interests diverge. He sniffs at your music collection and refuses to grasp how "someone of your intelligence" can watch Strictly or I'm a Celebrity. As he sits in the kitchen shouting out the answers to University Challenge, you feel a pang of pity for The Male Condition. And then you feel annoyed.