How truthful are people when answering surveys?

In the past I have conducted surveys on issues around sustainability and using technology to extend the capabilities of the body, whenever I presented the result people questioned the validity of the data because most respondents had very positive attitudes on these issues. "They just want to look good," everyone says. Interestingly, I came across this study that found that most respondents are more likely to give honest answers to a self-administered questionnaire compared to an interview questionnaire. More so, young people people are more likely to be honest when responding to a computer based questionnaire (a tool that I use often) (Turner, Ku and Rogers, 1998)