Alright. Let’s go ahead and argue about the study that found even more lists on which to put Rhode Island at the wrong end, such as neurosicness and disagreeableness:

Led by scientist Peter J. Rentfrow of the University of Cambridge, England, the researchers ranked the states and the District of Columbia in each of the five dimensions of personality (the so-called Big Five) that psychologists commonly use to describe individual humans: Openness, Conscientiousness, Extraversion, Agreeableness and Neuroticism.
Rhode Island ranked poorly on all.
The state was judged 40th on extraversion, characterized by positive emotions and the desire to seek the company of others; a near-bottom 48th on conscientiousness, which involves self-discipline and organized planning; and a middling 28th on openness, an appreciation for adventure and imagination. The state was ranked 45th on agreeableness (a tendency toward cooperation and compassion), 6th from the bottom, but was number 2 on the one negative trait, neuroticism (a type of emotional instability).

I’m not sure I buy the analysis that the trait was handed down by cantankerous founders, although it strikes me as reasonable to believe that other historical and geographical factors are the primary cause that binds those founders with the modern day attitude. The state is a few turns off the direct path from New York City to Boston by land and is dominated by a natural harbor; such qualities probably affect the sorts of people who settled (and continue to settle) here.
Whatever the cause, I’ve heard the same commentary from too many transplants to discount the accuracy of the general impression. Just as friends who’ve spent extended periods in other parts of the country have commented about the weight and smell of the air when they first disembarked from the airplane, Midwesterners and Southerners who’ve found themselves in Rhode Island will sometimes mention, in unguarded moments, the few extra points that Rhode Islanders deserve on the meanness scale.
One of the guys from the Rhode Show (which doesn’t make it easy to find the hosts’ names) argues that it’s more an Eastern Seaboard thing — from Washington to Boston — than a Rhode Island thing, although the maps in the study itself suggest that the state’s region is only partly exculpatory (PDF). It’s telling that the host proceeded to defend — quite neurotically — the region’s disagreeableness as mostly indicative of honesty.
For my part, I’d add my impressions to those of the above mentioned fly-over staters, and I’m from New Jersey. (The relative scores of that state and Rhode Island seem pretty accurate, to me.) I’d also point out that honesty only comes across as mean when one is inclined to think mean thoughts, although I’ll concede that it’s a coastal quality to voice the intellectual conceit that those who behave more politely are merely pretending.
Some mitigation must be acknowledged — for the sake of fairness — in the fact that a state’s long decline will inevitably have an effect on the attitude of its population. With that, though, we face an impossible chicken and egg.