Moody people are often baffling to those of us with steadier emotions.
喜怒無常的人常常令情緒穩定的人感到困惑。

But scientists argue that changing moods – as seen in sulking teenagers, grumpy spouses, or bad-tempered parents - serve an important purpose.
但科學家認為情緒多變——多見于陰郁的青少年，愛發牢騷的配偶，或者暴脾氣的父母——其實有它的重要目的。

Rather than being a weakness, they are nature's way to of helping us adapt to an ever changing world. So when times are good and spirits are high, we take more risks at a time they are likely to be rewarded.
這不是缺點，而是幫助我們適應瞬息萬變的世界的自然方式。

And when times are tough, sulking can help us conserve our energy.
當萬事順意、志得意滿時，我們更傾向于去冒風險，因為我們覺得自己很可能會得到回報。當時局艱難時，陰郁的情緒可以幫我們保存能量。

To take a human example, a stock market trader who makes a successful deal becomes more optimistic about the outcome of his next transaction.
舉人類為例，股市交易者大賺一筆時，他對下一筆交易將會信心滿滿。

He is then likely to take more risks – as he becomes more optimistic things will go in his or her favour.
他會傾向于冒更多險，因為他樂觀地期待事情會順遂心意。

The increase in risk taking allows him to make maximum gains at a time when risk taking is most likely to pay off.
在風險投資回報率高時，冒更大風險讓他可以獲得最大化的利益。

This holds true when a variety of different events have an underlying connection, the authors argue.
作者論證說，當多個不同事件之間存在潛在聯系時，這個結果是成立的。

To take an example from the natural world, the authors suggest an animal's mood improves on finding a number of fruits in a tree.
作者又舉了一個自然界的例子，他指出動物在一株樹上發現許多果實時情緒會改善。

Rather than assessing each tree individually, this good mood helps the animal to look at the big picture – and optimistically make it think fruits are in abundance.
情緒大好的動物不會挨個打量每株樹，而是會樂觀地認為這一片樹的果實都很豐碩。

This encourages the animal to climb up the neighbouring branch – where good conditions make it more likely that there will be a tasty fruit available.
這會鼓勵它爬上旁邊那棵樹，由于這片樹生長環境良好，所以那顆樹的枝頭上很可能掛著一個好吃的果子。

It may be increased rainfall or sunshine has caused fruit to become more abundant, the authors suggest.
作者指出，也許是降雨和日照增多讓果實更豐饒。

'In this situation, it makes little sense to update expectations for each tree independently,' the authors write, and makes the animal expect 'a general increase in reward and update expectations for all related trees accordingly.'
作者寫道：“在這種情況下，動物不會對每棵樹都抱有不同的期望，而是會整體地期待‘周邊的樹都能有更多回報’。”

Eran Eldar, of University College London said: 'This effect of mood should be useful whenever different sources of reward are interconnected or possess an underlying momentum.
英國倫敦大學學院伊然·艾爾達說：“當不同回報的來源相互間都有聯系或擁有潛在動力，這種情緒效應將會很有用。

'That may often be the case in the natural as well as in the modern world, as successes in acquiring skills, material resources, social status, and even mating partners may all affect one another.'
“不僅在自然界，在現代社會，獲取技能的成功與獲得物質資源、社會地位，甚至和找對象都是環環相扣的。”

When rewards are starting to thin out – for example if winter is coming – then a negative mood will result in an individual event, such as finding a fruit, being 'downgraded'– allowing behaviour and expectations to be quickly adjusted downwards.
當回報減少——比如冬天來臨——消極的情緒就會致使動物對找到果實的期望降低，從而調整自己的行為。

The authors suggest that this could lead to behaviours such as an animal deciding to save energy and hibernate instead of rushing around.
作者說，這會導致動物決定節省體力或冬眠，而不是到處亂跑。

英文來源：每日郵報
譯者：劉傳坤
審校&編輯：丹妮