The 5 principles I used to pick my wife's 40th birthday present
1. Anticipation increases happiness. (Applied Research in Quality of Life). The study showed that the greatest boost in happiness of a vacation came before the vacation.
2. Spending money on experiences has a much bigger impact on happiness than material goods. (Leaf Van Boven and Thomas Gilovich)
3. There is a strong correlation between quality of people's relationships and their happiness (just Google it - massive number of studies). Activities that lead to stronger social bonds between friends and family are likely to lead to last increases in happiness.
4. "Hedonic adaptation" is where you rapidly get used to changes. It's harder to adapt to experiences because they are typically so many small aspects to them, as opposed to turning on a new TV or starting a new car. However, you can adapt to experiences as well. It's best to break up experiences as well to avoid adaptation. More frequent experiences as opposed to one big one maximally prevents adaptation.
....finally if all doesn't work out just right - (5) it turns out that you remember your experiences fondly regardless of what actually happened. In the NYTimes, Sonja Lyubomirsky, a psychology professor at University of California, Riverside said that experienes are typically "airbrushed" with "rosy collection." Your mind forgets the bad moments over time. There clearly is a huge evolutionary advantage in this.
Can anyone guess the present?