Giving to others not only makes us happier but can help lower our blood pressure, while humans are most miserable in their forties for deeply rooted biological reasons, researchers told a conference in London on Thursday.

Experts from Britain and Canada were debating the latest research on happiness and altruism, as part of a conference on "nudging" - using psychological insights into human behavior to get people to make good decisions.

Happiness is highly subjective, and cultures have very different conceptions of what it means, making it difficult to study and measure - but an increasing number of organizations think it is an important subject to focus on.

The United Nations uses a Human Development Index, which incorporates factors like life satisfaction alongside more conventional economic metrics like gross domestic product (GDP), and publishes a World Happiness Report each year.

THE MONKEY PUZZLE

Happiness over an adult lifespan is "a giant U shape," high in our 20s, low in our 40s, and high again in old age," said Andrew Oswald, an economics professor at Britain's University of Warwick.

He published this conclusion several years ago, but has since carried out similar studies around the world, all of which reveal the same conclusion - that happiness follows a U shape, and depression follows a 'hump,' peaking in the 40s.

"The standard theory was one of thwarted aspirations," he said on Thursday, with people realizing as they approach middle age that they are unlikely to realize the dreams of their youth, and getting used to this in later life.

However, social theories like this do not explain an observation from an unusual source: zookeepers.

Animal experts who observe great apes, assessing their psychological wellbeing over time, say that monkeys - like humans - are most likely to be sad in middle age.

This suggests a midlife crisis may be "intrinsically natural," but it is a "puzzle which remains to be understood," said Oswald.

GIVERS ARE HAPPIER

People who give money to charity tend to be happier and also healthier than others, said Elizabeth Dunn, a psychology professor at the University of British Columbia in Canada, where she studies happiness.

"Money doesn't seem to buy quite as much happiness as many people assume," she told the conference, adding that in experiments where people were given money and told to spend it on either themselves or others, givers were happiest afterwards.

Similar results were reached in Canada and Uganda, two hugely different countries in terms of income and culture, suggesting this may be something intrinsic to humans, she said.

"People who donate money to charity are happier in poor and rich countries alike," she said. "You don't have to have a lot to experience the emotional benefits of giving."

As well as the link between happiness and altruism, which she now thinks is fairly robust and well established, Dunn said her more recent research suggests giving money away can tangibly improve one's health.

People who donate to charity have lower blood pressure, she said, even when controlling for factors like income, wealth, age and exercise, which suggests the giving itself is responsible.

Dunn measured people's blood pressure before and after giving, and found it fell when people gave significantly to other people or causes, but did not change when they spent money on themselves.

The extent to which people feel connected to the cause is also important, she said, with more of an effect when people feel personally connected to the cause to which they are giving.

"Giving is "not just heartwarming, it may be quite literally good for our hearts," she said.