On August 25, 74 -year old Tom Robb went for a walk and disappeared, although teams of local searchers combed the country for him.

The Stirling farmer would now be 75.

The contrast between the media interest in Mr Robb and the national coverage of the lost child April Jones is stark. If the media response to people going missing can make a difference to whether they are found, why do we leave it to the media to decide which cases will have relentless exposure, and which will struggle to be noticed?

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A "hierarchy of grieving" is an expression describing how some people are more and some less acknowledged as entitled to be distressed.

The pain of the parents of a missing child who are consumed with fear cannot be imagined. The anguish of the crowds who pile flowers or toys at a makeshift shrine is undoubted. But their feelings have been generated and heightened by the media. TV pushes complete strangers up the hierarchy of grieving until they feel personally affected. An important and difficult question to ask is why the media is interested particularly in this child.

The website of the charity Missing Kids shows photographs of children who are not so widely known. Looking at their faces the disagreeable idea occurs that they don't count so much because most of us don't identify with missing ethnic minority children.

When people turn out in hundreds to search, they are doing two things. They are distracting themselves from anxiety, by doing something practical. They are also fulfilling the social contract which says we should care about all children and share responsibility for their protection.

Evidently this contract is more closely observed when the "social" relationship is strongest.

But what does the social contract say about older people? In some countries there is a Seniors Alert System, which follows the pattern of the Amber Alert for children in the US. When an older vulnerable person goes missing, police send out an alert more quickly than for other adults.

A UK version of this is being trialled in Oxfordshire, using volunteers and the Neighbourhood Watch to act swiftly when a person with dementia wanders. This is based on a highly successful Belgian programme that is recognised across Europe. Manufacturers are racing to develop affordable electronic locator devices, as discrete as a wristwatch and families will pay large sums to obtain them for their elders.

Even local authorities are investing in these, recognising that they delay the day when the person is put in a care home "for their own safety".

But when an old man or woman goes missing, it's not big news and there is not usually a large crowd translating their anxieties into the busy work of searching.

Is this because the social contract that says we must care for children who are vulnerable does not cover adults who are vulnerable? Is the sort of person who edits news broadcasts indifferent to the needs of old people? Distraught families feel that after any initial publicity no-one cares.

When anyone disappears we get stuck in a permanent state of search. The need to care for that child persists long after the physical presence of the child is gone.

Maybe with older people, people find easier to move away from the need to search. Maybe they tell themselves that the old person was close to death anyway. Life goes on, and whatever happened, they are probably not suffering any more and we need to move on and get over it. That's one explanation.

But we must not ignore the possible belief that a person with dementia was not worth looking for, because we all fear dementia so much. A recent poll of people who'd had their dementia "awareness" raised said they were more aware but they'd rather have cancer.

If TV has so much power over our thinking, it is time some effort was made to use that power to give a more positive message. In any case, our collective failure of empathy for the grieving families is shocking.

Professor June Andrews is head of the Dementia Services Development Centre at the University of Stirling