Passenger profiling is controversial but used in varying forms worldwide.
UK airports operator BAA says selection of those singled out for questioning is "completely random", but other countries use profiling to cope with sheer numbers.
Staff are trained to spot those fitting criminal profiles ranging from drug-runners and illegal immigrants to terrorists.
And travellers at an increasing number of airports are watched by staff who can recognise suspicious or anxious behaviour, like changes in mannerisms, sweating or changes in the pitch of voices.
The new techniques, trialled in the US since September 11, run alongside established checks that flag up those on passenger "watch lists".
Time magazine reported the Spot programme - or Screening Passengers by Observation Techniques - had been a success in a three-year trial at some of the US' busiest airports.
Spot does not use racial or ethnic factors, unlike Israeli airline El Al which has a policy of singling out young Arabs. The airline is quick to point out it has not had a hijacking in 30 years.
Human rights groups have criticised methods using gender, age, ethnicity and appearance.
But Sir Rod Eddington, former head of BA, has argued for profiling, asking whether it is logical for a grandmother to face the same scrutiny as a 25-year-old male.