There is a school of thought that falling crime levels must be down to massaging of the figures, or some other statistical sleight of hand.

But the latest data from the Scottish Crime and Justice Survey (SCJS) is just the latest confirmation of a now convincing picture. Overall levels of crime are falling and this seems to be a clear trend over time.

The biennial SCJS study asks people whether they have been a victim of crime, and other questions about their perceptions of the general crime rate in their area. It provides a reasonable, independent assessment of whether crime is more or less common.

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One in six adults is estimated to have been the victim of at least one crime over the past year: a drop, but also a marginally lower figure than that in similar studies south of the border.

This so-called "victimisation rate", based on self reporting, is 16.9% in Scotland, compared with 18.7% in England and Wales.

This is just one measure, however. Previous figures show reported crime is down, while other measures such as intelligence from hospitals about the fall-out from violent crime, patchy as it is, demonstrate the same trend.

It is important to acknowledge and welcome this, as well as the data from the SCJS report about people's level of confidence in the criminal justice system. A high proportion of respondents are confident it can bring justice for victims of crime, and can also offer the accused a fair trial.

There is still a question about whether people believe the evidence about falling crime. Respondents to the latest survey were more likely to think they will experience crime in future than is actually the case.

In some cases this is marked, with 7% of people fearing a housebreaking, compared to the 1.2% who will actually experience one.

The most interesting question, however, is not about whether perceptions of crime are as low as the reality. It is why the declining trend is occurring at all.

Police Scotland would say it is related to their methods and tactics. But falling crime rates cannot be related completely to what the police are doing.

The prominence of stop and search, increased use of CCTV and more effective disruptive tactics against known criminals might also all play a part.

But crime is falling in England, too, where forces numbers have had less protection and different methods have been deployed. Meanwhile, the trend is seen in other parts of the developed world: North and South America, and Europe, east and west.

The unexplained international trend is a puzzle some of the leading lights in criminology are still trying to understand.

It is not merely of academic interest, either. We should all be concerned to find out why crime is falling.

The worrying thing about it is that, if we do not know why it is happening, we do not know how to make sure it continues. This welcome development is by no means guaranteed to continue.