In Europe, formal futures markets appeared in the Dutch Republic during the 17th century. Among the most notable centered on the tulip market, at the height of tulip mania. At the peak of tulip mania, in February 1637, some single tulip bulbs sold for more than 10 times the annual income of a skilled artisan. Research is difficult because of the limited economic data from the 1630s, much of which come from biased and speculative sources. Some modern economists have proposed rational explanations, rather than a speculative mania, for the rise and fall in prices. For example, other flowers, such as the hyacinth, also had high initial prices at the time of their introduction, which then fell as the plants were propagated. The high asset prices may also have been driven by expectations of a parliamentary decree that contracts could be voided for a small cost, thus lowering the risk to buyers.