What is it that most makes us who we are? A study of people with Alzheimer's disease and other debilitating conditions suggests that individual uniqueness comes from one's particular moral behavior and when that changes, people no longer see the person as being the same.

As diseases such as Alzheimer's progress and worsen over time, they can reach a point where friends and loved ones say the sufferer no longer seems like themself any more and is an entirely different person. But what specifically are the changes that cause them to think so?

It's not a decline in mental faculties or memory that makes Alzheimer's and other patients seem to be entirely different people, it's changes in their moral behavior.

The researchers set out to explore this by asking friends and family members of people with Alzheimer's disease, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) about their observations over time.

In FTD, damage to the front or temporal lobe of the brain causes it to shrink or atrophy. This nerve loss is generally accompanied by extreme changes in personality and behavior. Unlike typical dementia, whose symptoms may improve or disappear over time, in FTD symptoms inevitably worsen.

ALS was included as a control condition. While it involves nerve degeneration, its effects are mainly on muscle function, while Alzheimer's disease and FTD patients usually show cognitive and personality changes.

The 248 volunteers, mostly spouses or partners of the patients, reported the degree to which their loved one showed symptoms of their disease. They also reported how much their family member had changed over time in a wide variety of different traits such as honesty and adventurousness.

Finally, they indicated how much they perceived the patient's identity as having changed as a result of the disease, answering questions like “Regardless of the severity of the illness, how much do you sense that the patient is still the same person underneath?”

As expected, people with FTD and Alzheimer's disease were more likely to show identity change than those with ALS. These changes in identity tended to be tied to caregivers' perceptions that the person was no longer as honest or truthful as he or she had been; they could no longer trust the person to be accurate. In other words, changes in traits related to morality, such as honesty and integrity, were key to how like themselves they seemed; but not other traits such as sociability and adventurousness.

Neither depression nor amnesia nor changes in other personality traits had an observable impact on perceived identity change. Only aphasia, the loss of ability to understand or express speech, showed any association with identity change, with aphasia's association being much weaker than that of change in moral traits.

These findings may also have implications for people who don't have a neurodegenerative disease. The way people see us depends largely on our moral behavior. And who hasn't been tempted to do things they know aren't right or to cut a corner here and there? One way to rationalize such behavior is to say that no one will ever notice. The study findings strongly suggest that people do indeed notice and will see us differently once we start ignoring our moral imperatives.

The study appears in Psychological Science.