By Dominic Casciani
Home affairs reporter, BBC News
When is a gang a gang?
The fatal shooting of 11-year-old Rhys Jones in Liverpool has once again brought to light the issue of gun and knife crimes being carried out by gangs in Britain's cities.
What do we know about gangs in the UK?
Very little. Criminologists have researched gangs for decades - but part of the problem is that the word gang itself can be misleading.
So what do we mean by the word gang?
It depends. Firstly a gang, from a policing perspective, means a group of people involved in crime. But a gang can also mean a group which provides emotional or psychological support to its members. These two definitions are not exclusive.
WHAT IS A GANG?
THREE "TIERS OF RISK"
Lowest: Peer groups
Middle: Street gangs
Highest: Criminal networks
Source: Home Office definition used by its Street Gangs Expert Group
Then there are other factors. Some gangs may be territorial - such as those involved in selling drugs. Others may be based on ethnic considerations or simply attached to an estate where the members live.
Critically, a Youth Justice Board (YJB) study recently warned we should not over-use the word gang because it glamorises minor anti-social behaviour and encourages more serious criminality.
This is particularly important in relation to "wannabes" - teenagers on the fringes of offending who are seduced by some of the symbolism associated with gang culture.
The Home Office uses a definition based on "three tiers of risk" - with some gangs worse than others.
But gangs do play a role in youth offending?
Criminologists tend to say that most young people who break the law do so in a group environment. If you are emboldened by friends it becomes easier to think you can get away with it.
Two thirds of Youth Offending Teams - the local units tasked with rehabilitating young offenders - told the YJB they knew of "troublesome youth groups". But there may be a world of difference between these groups and well-organised criminal "gangs".
What kind of crimes are we talking about?
Drugs are at the heart of the most serious concerns about gang culture.
Professor John Pitts of the University of Bedfordshire has studied gang membership in parts of London. He found the fundamental change in gang culture had been the development of the drugs market. Drugs are a major business which requires an "expanding workforce" to keep the goods moving to addicts.
And the gang fulfils that role?
Younger gang members protect a geographic patch, ensuring efficient delivery of the drug itself and protecting the senior figures from detection.
The street gang, in the words of one leading sociologist, becomes the "shop floor of the international drugs business".
Are these teenagers all willing participants?
Professor Pitts found that 40% of the young people he came across in his research were "reluctant gangsters" - teens with no criminal record.
Some of these kids are scared of violent reprisals if they leave a gang or disagree with its aims. Affiliation becomes a form of reluctant protection for their family.
How many gangs do we think exist?
There have been reports of 257 crime-linked youth gangs in London alone - but the Metropolitan Police says it doesn't recognise the figure.
Experts say it is very difficult to put a number on these things because groups constantly change, rename themselves, reform or break-up. What is perhaps more important is to look at who is affected.
Prof Pitts' research suggests that the activities of a hardcore of 40 gang members running a serious crime enterprise can ultimately affect the lives of 6,000 people.
Where do weapons come into the equation?
Again it's very difficult to say. A Home Office study published in 2004 looked at arrested adults who declared gang membership.
It found an increase in the numbers claiming to carry any weapon, but little change in the number claiming to have carried a gun. Experts think the increase is down to more knives being in circulation.
What drives membership?
Gangs develop in specific local circumstances - but key factors appear to be family problems leading to dependence on friends and school problems, if not exclusion itself. The YJB study found that gang membership came after a slow process which began with anti-social behaviour as a child, influenced by older boys.
Is there kinship in gang membership?
The gang environment, say experts, can provide the "respect" that the members crave - essentially self esteem amid seeing themselves as outcasts. The YJB found gang members saw other advantages, particularly protection and kudos.
Are gangs a serious problem?
Some research is now pointing towards similarities between British gangs and the style of US gangs - namely the role of drug dealing.
The question is whether these changes mean that youth street crime is fundamentally changing from minor offending or anti-social behaviour to something more sinister and difficult to stop.