Today more than half of the world population has fertility below the replacement level, i.e., less than two surviving children per woman; and there are now several countries where fertility has dropped to levels that are of serious concern to policy-makers. All major population projections currently assume that fertility in the countries with the lowest levels will recover or at least not decline any further. Should birth rates, however, defy these projections and continue to decline, then these populations would shrink and age much more rapidly than currently assumed. The "low fertility trap hypothesis"  discussed in this contribution gives plausible reasons why indeed fewer and fewer people may want to have children in the future.