Michael Shermer in Scientific American:
“Could switching to Geico really save you 15 percent or more on car insurance? Was Abe Lincoln honest?” So intones the Geico commercial spokesperson, followed by faux vintage film footage of Mary Lincoln asking her husband, “Does this dress make my backside look big?” Honest Abe squirms and shifts, then hesitates and, while holding his thumb and forefinger an inch apart, finally mutters, “Perhaps a bit,” causing his wife to spin on her heels and exit in a huff.
The humor works because we recognize the question as a disguised request for a compliment or as a test of our love and loyalty. According to neuroscientist Sam Harris in his 2013 book Lying (Four Elephants Press), however, even in such a scenario we should always tell the truth: “By lying, we deny our friends access to reality—and their resulting ignorance often harms them in ways we did not anticipate. Our friends may act on our falsehoods, or fail to solve problems that could have been solved only on the basis of good information.” Maybe Mary's dressmaker is incompetent, or maybe Mary actually could stand to lose some weight, which would make her healthier and happier. Moreover, Harris says, little white lies often lead to big black lies: “Very soon, you may find yourself behaving as most people do quite effortlessly: shading the truth, or even lying outright, without thinking about it. The price is too high.” A practical solution is to think of a way to tell the truth with tact. As Harris notes, research shows that “all forms of lying—including white lies meant to spare the feelings of others—are associated with poorer-quality relationships.”
More here.