Objectives: Performance on neuropsychological tests can be influenced by non-cognitive factors, including deliberate underperformance, stress, the need to fulfil a sick role, depression, un-cooperativeness, fatigue and unhappiness with the evaluative situation. Tests to detect suboptimal effort are becoming widely used in clinical practice and are based on their reported insensitivity to cognitive dysfunction. A diagnosis of dementia has life-changing implications for an individual of working age. It is therefore crucial that clinicians can be confident they have obtained a valid estimate of current cognitive functioning. This study aimed to establish whether mood or cognitive functioning adversely influenced performance on symptom validity tests (SVTs) in individuals with working age dementia, who were judged to be using full effort. Method: Forty-two participants with dementia diagnosed before the age of 65 completed measures of emotional and cognitive functioning and six SVTs. Results: There were no significant correlations between emotional functioning and measures of effort. However, cognitive functioning, in particular new learning, was significantly related to SVT performance. Participants with mild dementia performed significantly better on all six SVTs than those with moderate/severe dementia. The results also suggest that the SVTs are not equally sensitive to cognitive dysfunction. Rey's Dot Counting Test (DCT) (times criterion) achieved 100% specificity, and was the only test passed by every participant. Conclusion: Rey's DCT is the only SVT that can be recommended for use in those under 65 years with possible dementia at this time until further validation studies are undertaken.