ecent scientific studies have shown that happiness is not determined by genetics and can be controlled. Scientists from the University of California last year reviewed 51 studies that tested attempts to increase happiness through different types of positive thinking, and found that these practices can significantly enhance well-being. The results were published in the Journal of Clinical Psychology.

Here are five things that research has shown can improve happiness:

1. Be grateful - Some study participants were asked to write letters of gratitude to people who had helped them in some way. The study found that these people reported a lasting increase in happiness - over weeks and even months - after implementing the habit. What's even more surprising: Sending the letter is not necessary. Even when people wrote letters but never delivered them to the addressee, they still reported feeling better afterwards.

2. Be optimistic - Another practice that seems to help is optimistic thinking. Study participants were asked to visualize an ideal future - for example, living with a loving and supportive partner, or finding a job that was fulfilling - and describe the image in a journal entry. After doing this for a few weeks, these people too reported increased feelings of well-being.

3. Count your blessings - People who practice writing down three good things that have happened to them every week show significant boosts in happiness, studies have found. It seems the act of focusing on the positive helps people remember reasons to be glad.

4. Use your strengths - Another study asked people to identify their greatest strengths, and then to try to use these strengths in new ways. For example, someone who says they have a good sense of humor could try telling jokes to lighten up business meetings or cheer up sad friends. This habit, too, seems to heighten happiness. 5. Commit acts of kindness - It turns out helping others also helps ourselves. People who donate time or money to charity, or who altruistically assist people in need, report improvements in their own happiness.