Gardai investigating a possible serial killer will be looking for "regularity" in the cases, according to Prof Max Taylor, of the applied psychology department of University College Cork.
Prof Taylor pointed out that the list of women who had disappeared immediately displayed two signs of regularity in that there was a cluster in the Dublin-Wicklow-Kildare area and that the women tended to disappear at certain common times of year, particularly November-December, July and February-March.
"It looks as though the nearer you get to this central area (Dublin-Wicklow-Kildare) incidents tend to take place in July, November-December and February-March." Prof Taylor also said there was some sign of the number of disappearances "accelerating" in that there appeared to have been only one or so every couple of years but now they were occurring at about two a year.
He said the investigators would be looking for other more important signs of regularity, including the way the victims died and how they were sexually assaulted.
The women whose bodies have been recovered are understood to have been strangled or beaten to death.
There have been instances of serial killing here before. The first known figures who could be termed serial killers are John Shaw and Geoffrey Evans.
Shaw and Evans came to Ireland in 1976 from the north of England, where they were being sought by police for three rapes. They determined to kill a woman every week while in Ireland. They killed two women, Ms Mary Duffy (25) from Mayo and Ms Elizabeth Plunkett (23) from Dublin, before being caught by gardai. Shaw and Evans, now in their late 50s, were convicted of murder and remain in Arbour Hill Prison.
The known indigenous multiple killers tend to engage in killing rampages rather than systematic abduction and murder. Brendan O'Donnell, who died in prison last year, kidnapped and murdered Ms Imelda Riney, her three-year-old son Liam and Father Joe Walsh in a two-day burst of violence in east Clare in May 1994.
There are residual suspicions that the English paedophile serial killer, Robert Black, who is serving life imprisonment for killing three young girls in the north of England and Scotland, may have killed here.
There is strong suspicion that Black, a van driver who visited Ireland while working for a poster delivery company, killed Jennifer Cardy (9), the Co Down girl whose body was found in a dam near Hillsborough six days after disappearing in August 1981. The manner of her death bore strong similarities to those of Black's other victims.
There is suspicion - but no evidence - that Black might also have been involved in the disappearance of Mary Boyle (12) near Ballyshannon, Co Donegal. She disappeared from her grandparents' home in March 1977 and has not been seen since.
