The single most important thing to do today if you want to live a long, happy life

By allowing unobserved individual fixed effects to be factored out from the life satisfaction equation, an increase in the level of social interaction with friends and relatives is estimated to be worth up to an extra £85,000 a year. In terms of statistical significance, this is strikingly large. The estimated figure is even larger than that of getting married (which is worth approximately £50,000). It can compensate for nearly two-third in the loss of the happiness from going through a separation (minus £139,000) or unemployment (minus £143,000). It is also roughly nine times larger than the average real household income per capita in the dataset, which is around £9,800 a year.

According to the American Psychological Association, the most effective stress-relief strategies are exercising or playing sports, praying or attending a religious service, reading, listening to music, spending time with friends or family, getting a massage, going outside for a walk, meditating or doing yoga, and spending time with a creative hobby (The least effective strategies are gambling, shopping, smoking, drinking, eating, playing video games, surfing the Internet, and watching TV or movies for more than two hours).

Is there someone in your life whom you would feel comfortable phoning at four in the morning to tell your troubles to? If your answer is yes, you will likely live longer than someone whose answer is no. For George Vaillant, the Harvard psychiatrist who discovered this fact, the master strength is the capacity to be loved. Conversely, as the social neuroscientist John Cacioppo has argued, loneliness is such a disabling condition that it compels the belief that the pursuit of relationships is a rock-bottom fundamental to human well-being.

"…the only thing that really matters in life are your relationships to other people."

In one experiment, Cacioppo looked for a connection between the loneliness of subjects and the relative frequency of their interactions via Facebook, chat rooms, online games, dating sites, and face-to-face contact. The results were unequivocal. "The greater the proportion of face-to-face interactions, the less lonely you are," he says. "The greater the proportion of online interactions, the lonelier you are." Surely, I suggest to Cacioppo, this means that Facebook and the like inevitably make people lonelier. He disagrees. Facebook is merely a tool, he says, and like any tool, its effectiveness will depend on its user. "If you use Facebook to increase face-to-face contact," he says, "it increases social capital." So if social media let you organize a game of football among your friends, that's healthy. If you turn to social media instead of playing football, however, that's unhealthy.

"Friends that we feel ambivalently about raise our blood pressure more — cause more anxiety and stress — than people we actively dislike."