1. Inflation
Gold prices rose 18-fold in the Seventies, while inflation hit peacetime records. But although inflation eased back over the next 20 years, it continued to eat into the purchasing power of cash, halving the dollar's real value by 2000.
But gold more than halved as well, losing 80pc of its value in real terms. Since then gold has risen by 350pc against the slowest official US inflation in half a century.
Over the past 45 years, gold prices and the US consumer price index show an average "12-month correlation" – a statistical measure of how closely two numbers move together – of precisely zero.
• Read more: How to invest in bullion
2. Interest rates
Because gold pays no interest, anyone who buys it "loses" the money that he or she would otherwise have earned in interest on their cash. This is called the "opportunity cost" of owning gold and clearly is greater when interest rates are high.
But while high interest rates make owning gold less attractive, there is no constant relationship with prices. In fact, gold prices and interest rates have moved in opposite directions only half the time since 1969. The rest of the time they rose or fell together.
3. Stock markets
As with interest rates, gold moves in the opposite direction to the stock market just less than half of the time, in fact in some 48pc of all 12-month periods since 1969. What's more, gold's 12-month correlation with the broadly based S&P 500 index over the past 45 years averages zero.
Gold offers as near a perfect non-correlation as equity investors wanting to diversify their holdings could wish for. This explains its value as part of a balanced portfolio of different types of investment, where what you want to avoid is all assets rising or falling together.
4. Geopolitics
Gold famously peaked in 1980 at $850 an when the Soviet Union invaded Afghanistan, and also coincided with the Iranian hostage crisis at the US Embassy in Tehran.
The rise to 2011's record peak of $1,920 came as several of the Arab Spring revolutions descended into civil war, Greece was brought to a standstill by a general strike against the Eurozone's austerity demands, and England suffered its worst rioting of modern times.
But while speculators who trade in gold "derivatives" often do move the price by raising their betting on breaking news, such moves tend to be brief.
For example, gold jumped by 12pc in the first two weeks of the Falklands crisis of 1982, but the deeper trend remained down, and those gains were erased by the end of May; the price hit a new three-year low after Britain retook Port Stanley in June.
Gold again jumped by $10 to $380 when Iraq invaded Kuwait on 2 August 1990. But that proved the average price throughout the war, with an early spike to $412 fading to $365 by end of the war seven months later.
By contrast, gold's fastest one-month gains of the past 30 years came in October 1999 (a 25pc rise, although prices were unmoved on the day both by Pakistan's bloodless coup and by the Russian apartment bombings that killed 293) and in May 2006 (a 23pc rise, but again unmoved by the major geopolitical news, Iran announcing its uranium enrichment programme).
5. The dollar
Like most natural resources, the gold price is typically quoted in US dollars. But the aphorism that a weaker dollar must make gold rise, and vice versa, applies only 60pc of the time. Over longer periods it can also mask gold's larger moves for investors outside the US.
Priced in sterling, for instance, gold's rise since 1968 has been 40pc greater than its rise against the dollar. Over the past 10 years gold has gained 273pc for British investors against 255pc for US buyers – and its strongest gains came when the dollar was also rising.
Gold priced in sterling has risen by more than 5pc in 27 months since 2004. In 21 of them, the dollar also rose against the pound.
6. Oil prices
Because popular imagination associates gold with geopolitical strife, and so much strife stems from the world's key crude oil sources, the two commodities are often assumed to move together.
Certainly, gold and crude oil move in the same direction together more often than they move in tandem with stock markets or interest rates – a little over 60pc of the time on an annual basis since 1986.
Gold's recovery from 30-year lows at the start of this century also coincided with a long bull market in crude oil and other natural resources. But while this "commodity supercycle" pulled in new money from fund managers in a way the gold market was long used to, it turned sharply lower when the financial crisis hit, driving crude oil 80pc lower in the last six months of 2008.
Gold, by contrast, found its floor much sooner before continuing its bull market, taking its gains over the 10 years to July 2014 to 235pc against 140pc for crude oil.
7. Asian demand
Gold prices fell by 30pc in 2013 against all major currencies, including the Chinese yuan. Chinese households became the world's heaviest buyers of gold in 2013. Over the first half of 2014 gold prices then recovered a third of that loss, even though Chinese gold demand fell by a fifth compared with the same period last year.
In short, Asian demand clearly follows prices, rather than setting them. Indeed, the former biggest market, India, was importing record quantities as prices crashed to three-year lows in spring 2013.
World prices then bottomed when India's government shut down legal imports in midsummer. The price then moved sideways before rising, despite a 75pc drop in Indian imports.
This is because, again, gold prices aren't driven by consumer demand – not from people who buy gold because it is gold. They tend to want more when prices fall, and vice versa. The people who count instead are investors moving out of other assets, choosing to buy gold because it isn't anything else and bringing in money which otherwise wouldn't hit the bullion market.
For a sustained rise in the gold price, you need a wall of professional money – as the bull markets of both the 1970s and 2000s showed.
Read more: Buy gold, they say – but how do you sell it?
In the round: gold is 'anxiety insurance'
What might boost or reduce the flow of investment money is a combination of all the factors above.
But at root, long-term rises or falls in the gold price reflect a broader anxiety – whether about politics, the value of money, or the outlook for other, typically more productive assets.
People tend to see gold as a form of financial insurance, and the cost of insurance is highest of course when you most need it.
Gold prices had already tripled and more by late 2008 from their 30-year lows of 2001. Demand for gold coins was so strong before the Lehmans bankruptcy that many retail dealers were already cleaned out by the time the investment bank failed.
The economic collapse stunned economists as well as the markets. Longer-term gold owners were less shocked. So too was their money.