Has the rate of sexual assault really increased in New Zealand since 2003?

By Joel Hellewell

This post is a response to the “meme” created by Nordic Model Now, claiming that the rate of sexual assault in New Zealand has increased since 2003, primarily because of the decriminalisation of prostitution in the Prostitution Reform Act 2003.

Link to the meme: https://nordicmodelnow.org/2016/08/11/meme-about-new-zealand-since-the-full-decriminalisation-of-the-sex-trade/

The first thing to do to the case numbers data should be to calculate the rate of sexual assaults and related offences by dividing the case numbers by the population of New Zealand (numbers just from google), this way we know that we are not seeing more sexual assaults because there are simply more people in New Zealand. The graph below shows the rates of recorded sexual assaults and related offences per 1000 people in New Zealand from 1994 to 2014. The Nordic Model Now meme uses data starting at the year 2000 “for simplicity”, it isn’t clear why this should be done considering that the data goes back to 1994. Including all the data from before 2000 does ruin the visual impact of their graph somewhat and makes the simple linear regression line less steep. The plot below shows a simple regression line for all years (1994-2014) in blue and a regression line for years 2000-2014 (used by Nordic Model Now) in red.

Hosted at: https://i.imgur.com/LmUCqlp.png

The biggest issue with this data, which is freely available from the New Zealand Statistics website, comes from the data footnotes for the year 2006.

“In June 2005, Police replaced the aging Law Enforcement System (LES), commonly known as the Wanganui computer, with a newer National Intelligence Application (NIA). This IT system migration is the largest crime-recording system change that has occurred since the introduction of the Wanganui computer in the late 1970s. This system change caused a step-increase in recorded crime statistics, coincident with the system replacement. This step -increase varied in magnitude between different crime-types and Police Districts. Caution should therefore be observed when making inferences from these statistics about trends the incidence of crime in New Zealand. “

(Link to the data footnotes: http://www.stats.govt.nz/tools_and_services/nzdotstat/tables-by-subject/new-zealand-recorded-crime-tables/release-notes.aspx)

Due to the police using a different computing system for reporting from mid-2005 onwards, the data is not comparable before and after this event since we don’t know to what extent the reporting system has had an impact of the number of cases. There is a definite jump in the rates of sexual assault and related offenses in 2005 and 2006, but to what extent is this a change in reporting or an increase in sexual assault? It is impossible for us to know.

What we can say is that there seems to be an increase in the rate of sexual assault and related offences since 2012, with a large increase in 2013. Reasonably, the footnotes caution that changes in education and awareness may change the likelihood of reporting, making it look like the rate of sexual assault is increasing when only the rate of reported sexual assaults is increasing (which is a good thing). There is nothing in the footnotes that mentions any change in legislation that may have caused this rise so sexual assault could unfortunately have risen in 2013.

Hosted at: https://i.imgur.com/3Fw3Sm4.png

The Nordic Model Now conclusion that the Prostitution Reform Act 2003 is related to a rise in sexual assault in implausible for several reasons:

  1. The data are incomparable before and after 2005 because of changes in the system of reporting of cases used by New Zealand Police
  2. Ignoring the previous issue, the upward trend in the rate of sexual assault seems to begin around 1999-2000, before the Prostitution Reform Act was even introduced in 2003.
  3. Looking at the data since 2005 that is comparable, the rate of sexual assault seems to have risen mainly between 2012 and 2013. It is unclear how or why the Prostitution Reform Act would have caused this rise 10 years after its introduction.

The rate of sexual assaults and related offences will correspond to the amount of effort and resources put into tackling them by the New Zealand police, as well as the climate in which victims come forward to report these offences. It’s very hard to show that any rise in the rate of sexual assaults and related offences is solely or even in part due to the Prostitution Reform Act.