Having constant negative thoughts over a long period of time may increase the risk of developing dementia, scientists believe.
Researchers at University College London said repetitive negative thinking (RNT) was linked to the deposit of harmful proteins in the brain which are associated with Alzheimer's disease.
The risks were also linked to long periods of RNT, rather than short-term negative thinking, they added.
"Depression and anxiety in midlife and old age are already known to be risk factors for dementia," said Dr Natalie Marchant, the lead author from the UCL division of psychiatry.
"Here, we found that certain thinking patterns implicated in depression and anxiety could be an underlying reason why people with those disorders are more likely to develop dementia.
"We expect that chronic negative thinking patterns over a long period of time could increase the risk."
The study of 292 people over the age of 55, which was supported by the Alzheimer's Society, was published in the journal Alzheimer's & Dementia.
Fiona Carragher, director of policy and influencing at the Alzheimer's Society, said: "Most of the people in the study were already identified as being at higher risk of Alzheimer's, so we would need to see if these results are echoed within the general population."