偶然翻閱英國金融時報博客欄，見到一則問答相當精彩，隨手摘錄下來了。舊日曾見歐美有雅士以經濟學解戀愛行爲，比之《魔鬼經濟學》所述靈活有趣得多。尤喜下文中關於經濟學的“沉沒成本（sunk costs）逝之固不足惜，沉沒得益(sunk benefits)既得又何足侍。”的論斷。妙哉斯言。女生問得癡情，教授答得老辣。成本是無可避免的最高代價。（the cost of an event is the highest-valued opportunity necessarily forsaken）。一切成本皆機會成本。真是“生無可恃，死不足惜。”

女生問：

親愛的教授， 我一直和男友住在一起。直到有一天，男友考上了倫敦的一所名校，他要我跟他一起去倫敦生活。我猶豫不決，但偷偷申請了倫敦的另外一所著名大學讀我夢寐以求的碩士學位，這件事我沒有馬上告訴他。後來我也順利被錄取了。男友在我得知被錄取前跟我分了手。當時他自己也猶豫不決，我們爭吵的很厲害。他說我拋下一切跟他去倫敦一起生活的話，他可能沒準備好負起這個責任。當然，當初讓我去倫敦也是他的主意。

現在他已經在倫敦小半年了，我剛到這裏。我仍然喜歡他，但我認爲他太自私了。我是否應該原諒他去找他呢？人類的自私和經濟學有着莫大的關係，我想聽聽您的意見。

經濟學教授答：

親愛的同學，

這段經歷對你而言不是壞處反而有好處。在你前男友的“鞭策”下，你堅持了自己的夢想並最終得以實現。太好了。

但是，在我看來，目前看似圓滿的結果並不能成爲你們重新開始的理由。論理，我會建議人們必須忽略“沉沒成本”，換句話說，就是所謂“覆水難收”。但在這件事情上，我卻要你忽略“沉沒得益”。忘掉目前看似圓滿的結果吧，把注意力放在這件事本身。如果當時你的男友是不理性的，或者說是一時衝動，那麼他是自作自受，罪有應得。更糟的是，如果你的男友是理性的，也就是說，當時的行動是基於得失的計算，那麼，作爲一個經濟學家我的意見是，他當時的建議無非是給自己提供一個方便罷了（讓女友跟自己來倫敦），同樣，一旦他發現女友對他已無價值，就會毫不猶豫的拋棄掉。我猜他當時可能覺得在倫敦會有更好的選擇吧。但毫無疑問，我倒覺得像你這麼聰明的姑娘，在這裏會有更好的選擇。

附上英文原文：

Dear Economist: Should I forgive my selfish ex-boyfriend?

I was living with my boyfriend when he got accepted to a very good university in London. He convinced me to go with him. I applied and got accepted to another very good university to pursue a masters degree that I long dreamed of. He broke up with me before I knew I had been accepted, arguing he wasn’t prepared to carry the responsibility of me leaving a life behind to go after him. Of course, it was his idea in the first place.

He has been living in London since September. I have just arrived. I still love him, but I think he has been very selfish. Should I forgive him and look him up? Human selfishness and economics have always walked hand in hand. I would love to have your advice.

CP

Dear CP,

The whole episode seems to have been very good for you: at your boyfriend’s urging, you followed your dreams and now you are realising them. Wonderful.

However, this happy fact does not translate into a compelling reason to renew the relationship. Normally I advise people that they should ignore “sunk costs” – to put it another way, there is no use crying over spilt milk. In this particular case you should ignore the “sunk benefits”. Forget the fact that it’s all worked out well, and focus instead on what this story tells you about the prospects for a happy future together.

If your boyfriend is not rational then his behaviour is self-destructive and self-obsessed. Even worse, if your boyfriend is rational – as economists usually assume – then he seems simply to have been opening up an option (to have his old girlfriend in London) and discarding it when it had no value to him.

I’d guess he thought he could do better than his old relationship in London. I’m quite sure that you can.

注：

CP是英文character pairing的縮寫，這個詞來源於日本同人圈。日文爲カップリング 或者カプ，表示人物配對關係。本意是指有戀愛關係的同人配對，近年來在其他場合也開始廣泛使用。