Which is to blame if a person is poor?

Since 1964, a number of surveys have asked “In your opinion, which is generally more often to blame if a person is poor: lack of effort on his own part or circumstances beyond his control?” (Since the 1980s, most have substituted "their" or "his or her" for "his").  The graph shows the difference between the percent saying "effort" and the percent saying "circumstances."  For example, the first time it was asked (February 1964), 34% said effort and 29% said circumstances, for a difference of +5%.  (Most of the others volunteered the opinion that it was some of both).  There are some short term changes--the most striking one is a strong move towards "circumstances" between 1988 and 1992 and a strong move back to "lack of effort" in 1994.  However, there is also a long-term trend towards "circumstances."  If you do a regression on time, the t-ratio is highly significant (3.62) and it remains significant even if you remove the outliers.  Many people believe that Americans have become more conservative on economic issues since the 1970s; others (including me) believe that they've stayed about the same.  From either point of view, the shift is surprising.