The lowest point of all came in 1975, at the end of a long decline during the Vietnam war,
with the fall of Saigon and America's humiliating defeat.
In Germany and Italy the first world war also caused dips in happiness.
By contrast, during the second world war these countries both got happier as the war continued.
Initially, that might be put down to their early successes,
but this can hardly explain German happiness when the Red Army was at the gates of Berlin.
The researchers hypothesise that what is being measured here is the result of propaganda and censorship,
rather than honest opinion. But they cannot prove this.
Earlier in Italian history, though, there was a clear and explicable crash in happiness in 1848,
with the failure of revolutions intended to unite into a single nation what were then half a dozen disparate states.
Surprisingly, however, successful unification in the 1860s also saw a fall in happiness.
As to wealth, the steady progress of the Victorian period matched a steady increase in British happiness,
as did the economic boom of the 1920s, which also lifted American spirits.
Both countries' spirits fell again in the Great Depression that followed the stockmarket crash of 1929.
After the lows of the 1970s, though, happiness in both has been on the rise ever since.
Overall, then, Dr Sgroi and Dr Proto found that happiness does vary with GDP.
But the effect of health and life expectancy,
which does not have the episodic quality of booms, busts and armed conflict, is larger,
even when the tendency of wealth to improve health is taken into account.
A one-year increase in longevity, for example, has the same effect on national happiness as a 4.3% increase in GDP.
And, as the grand historical sweep suggests, it is warfare that causes the biggest drops in happiness.
On average it takes a 30% increase in GDP to raise happiness by the amount that a year of war causes it to fall.
The upshot appears to be that, while increasing national income is important to happiness,
it is not as important as ensuring the population is healthy and avoiding conflict.