Moody¹ people are often baffling to those of us with steadier emotions.

喜怒无常的人常常令情绪稳定的人感到困惑。

But scientists argue that changing moods – as seen in sulking teenagers, grumpy spouses², or bad-tempered³ parents - serve an important purpose.

但科学家认为情绪多变——多见于阴郁的青少年，爱发牢骚的配偶，或者暴脾气的父母——其实有它的重要目的。

Rather than being a weakness, they are nature's way to of helping⁴ 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.

这不是缺点，而是帮助我们适应瞬息万变的世界的自然方式。

And when times are tough, sulking can help us conserve⁵ our energy.

当万事顺意、志得意满时，我们更倾向于去冒风险，因为我们觉得自己很可能会得到回报。当时局艰难时，阴郁的情绪可以帮我们保存能量。

To take a human example, a stock market trader who makes a successful deal becomes more optimistic about the outcome of his next transaction.

举人类为例，股市交易者大赚一笔时，他对下一笔交易将会信心满满。

He is then likely to take more risks – as he becomes more optimistic things will go in his or her favour.

他会倾向于冒更多险，因为他乐观地期待事情会顺遂心意。

The increase in risk taking allows him to make maximum gains at a time when risk taking is most likely to pay off.

在风险投资回报率高时，冒更大风险让他可以获得最大化的利益。

This holds true when a variety of different events have an underlying⁶ connection, the authors argue.

作者论证说，当多个不同事件之间存在潜在联系时，这个结果是成立的。

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.

作者又举了一个自然界的例子，他指出动物在一株树上发现许多果实时情绪会改善。

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.

情绪大好的动物不会挨个打量每株树，而是会乐观地认为这一片树的果实都很丰硕。

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.

这会鼓励它爬上旁边那棵树，由于这片树生长环境良好，所以那颗树的枝头上很可能挂着一个好吃的果子。

It may be increased rainfall or sunshine has caused fruit to become more abundant, the authors suggest.

作者指出，也许是降雨和日照增多让果实更丰饶。

'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.'

作者写道：“在这种情况下，动物不会对每棵树都抱有不同的期望，而是会整体地期待‘周边的树都能有更多回报’。”

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 momentum⁷.

英国伦敦大学学院伊然·艾尔达说：“当不同回报的来源相互间都有联系或拥有潜在动力，这种情绪效应将会很有用。

'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.'

“不仅在自然界，在现代社会，获取技能的成功与获得物质资源、社会地位，甚至和找对象都是环环相扣的。”

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 downwards⁸.

当回报减少——比如冬天来临——消极的情绪就会致使动物对找到果实的期望降低，从而调整自己的行为。

The authors suggest that this could lead to behaviours such as an animal deciding to save energy and hibernate⁹ instead of rushing around.

作者说，这会导致动物决定节省体力或冬眠，而不是到处乱跑。