You never know

 A few weeks ago, I wrote about a claim that "only about four percent of all marriages today are between a Republican and a Democrat."  I gave some results from surveys that asked people how their spouses voted, but mentioned that in some cases people might be mistaken.  There aren't many surveys that ask members of couples separately about their votes, but I found one from 1993 by Nancy Burns, Kay Lehman Schlozman, and Sidney Verba (data in ICPSR).  Among couples in which both members voted in the 1992 presidential election, 76% voted for the same candidate and 24% split their votes.  Of course, that election was unusual because an independent candidate (Ross Perot) got a large number of votes.  Limiting it to Bush and Clinton, 67% voted for the same candidate, and 7% split their votes.  

The survey didn't ask people how they thought their spouse voted for President, but they did ask whether they disagreed on any race.  In 47% of the couples, both said no, and in 22% both said yes.  In the other 31%, one partner thought that they didn't disagree on any races and one thought they did.  Breaking votes for president down by perceived disagreement:

                                 Same            Different

Both No                   96%              4%                      [89%-0%]
Split                         68%              32%                    [60%-15%]
Both yes                   46%              54%                    [32%-12%]

The figures in brackets are limited to Bush and Clinton votes.  Looking at it another way, almost half of the split votes, and more than half of the Bush/Clinton split votes, came from couples in which one member thought that they agreed on all the races (there were no clear gender differences in accuracy of perception).  

Although the data are old, I think that the general point is still relevant--if there's ambiguity, people tend to assume that their friends and family vote the same way that they do.  It's possible that increased political polarization has made people pay more attention to evidence about what their friends and family actually think, but it's also possible that it's made people more likely to avoid political discussions and more likely to assume that reasonable people agree with them.