While stocks have been climbing higher, fundamental indexing guru Rob Arnott of Research Affiliates hasn’t changed his outlook for the coming decade — and it’s not an optimistic one.

In a recent column for MarketWatch, Mark Hulbert looks at a forecasting model Arnott uses that has been highly accurate over the past century-plus. The model forecasts stock market returns by adding together just two factors: dividend yield and real growth in earnings and dividends. “Even if we generously assume that this latter growth rate will be just as high as in the past, despite the anemic economy, we only get a projected nominal return of between 5% and 6% annualized over the next decade [using the model] — about half stocks’ long-term average,” Hulbert writes.

According to the model, for stocks to post “anything close to [their] long-term average return, earnings and dividends will have to grow at more than double their historical average,” Hulbert says.

But Arnott actually thinks growth will be lower in the future than in the past, because of “demographic headwinds.” He thinks that could reduce GDP by 2% annually. Hulbert writes.