THE weight of money theory on Asian market performance is now very much a fact, as far as US flows are concerned. The latest numbers from the Securities Industry Association in the US have filled in some of the missing figures - and very impressive they are too. When the outflows in the final three months are finally added up, they are expected to show that US investors poured almost US$6 billion net into Hong Kong last year. The Asian story went much further than Hong Kong, however. In the first nine months, the total net cash coming into regional markets was $10.75 billion, but the territory was the hot favourite among US investors. The final quarter surge in other indices in the region will also be reflected in the substantially higher full-year figure. What the figures also show, however, is that the American appetite for Asian stocks has not overwhelmed its traditional tastes. The ''special relationship'' said to exist between the US and Britain might wax and wane, but American investors stay loyal to UK stocks, which remain their favourite international selection. Their appetite for Japanese stocks remained stronger than that for Hong Kong equities - but given the relative difference in the size of the markets, it was a modest helping. Where the flows go to now will be one of the keys to this year's performance. A warning of how rapidly one year's favourite can become next year's wallflower is embedded in the historic figures from US overseas investment. The overall Asian experience has been a switch-back: $7.32 billion in 1987 became a mere $1 billion in each of the following years - post-Crash and post-Tiananmen Square. The funds flooded back in 1991, with a staggering jump to $15 billion, before the total dived to $9 billion in 1992. It is true that Hong Kong has had a more consistent performance, but the lesson of fickle flows should have been learned. More encouraging is that there has not been a net sell-off recently, indicating a steady shift in asset allocation priorities. The domestic threat to international investment this year comes from a pick-up in the US economy, which will suck up spare liquidity which has been chasing equities. If that happens, one of the major factors in the bull market equation will have been removed.