THEY were innocent little boys from two separate families who died shockingly similar deaths of violence and torture at the hands of their mothers' de facto partners.

Four-year-old Bailey Constable and Jordan Anderson-Smith, aged just seven months, were killed more than a decade apart, but their deaths were no coincidence, according to a new study.

To be published later this year, the study on child abuse and protection will show de facto relationships put Australian children at increased risk, and male children aged under four years may be more likely to die a violent death.

Nathan Forrest was sentenced for bashing to death four-year-old Bailey Constable while bathing him in his home in Warren, NSW, on April 1, 2011.

Bailey's mother, Jessica Constable, who was not implicated in the death, said Forrest was injecting methamphetamine and was "like a ticking time bomb" on the final day of her son's life.

Last week, Bailey's family expressed outrage at Forrest's six-year minimum prison term, and an online petition has gathered more than 10,000 signatures for an appeal against the leniency of the sentence.

The case of Jordan Anderson-Smith has also undergone a court review as the baby's killer, Christopher Hoerler, applied yet again to the NSW State Parole Authority for release from prison.

Baby Jordan died following a party at which his mother, Louise Anderson, and de facto of two months, Hoerler, were drinking and smoking cannabis at their home in Wagga Wagga, NSW on February 25, 2000.

Hoerler beat the seven-month-old, who suffered a cut liver, bruised pancreas and small bowel, multiple broken ribs and facial injuries. So hard had Jordan been punched, his bottom teeth cut through his lip and palate.

The forensic pathologist who conducted the post mortem concluded the baby's "palate injury was caused by a blow to the jaw, and the abdominal injury by a non-penetrating blow or blows of considerable force".

Hoerler had also tortured the boy, crushing his toes almost certainly with a metal clamp. A coroner took four days to document the extent of the child's injuries.

Hoerler pleaded guilty to manslaughter and has served nine years of a 14-year maximum sentence. He was originally sentenced to a maximum term of 11 years with a non-parole period of eight years and three months. But the Court of Criminal Appeal, after hearing a leniency appeal by the Crown, increased the sentence to a maximum term of 14 years and four months and non-parole 10 years and nine months.

Ms Anderson, Jordan's mum, was not implicated in the death.

To this day, the case troubles the chief investigating detective and the dead boy's father, Cecil Smith, who was still undergoing counselling as he and his family braced for the decision on Hoerler's parole.

"I remember like it was yesterday," Smith told news.com.au.

He said he had become concerned about Hoerler showering with his infant son and other behaviour in the weeks leading up to the night the "unbelievable" happened and he went to his ex-girlfriend's house to find "two cop cars and an ambulance outside. They let me in to see him and I went and held him".

But according to a ground-breaking study by an Australian forensic psychologist, due out later this year, a number of risk factors make the deaths of Jordan Anderson-Smith, Bailey Constable and victims in other shocking child death cases entirely believable.

Lillian De Bortoli said children living with a single mother, on low household income, male children and children under the age of four were at greater risk of violence or death.

"Children who live with only one biological parent and one non-biological parent are at an increased epidemiological risk of becoming victims of physical abuse or homicide," she said.

"Australia-wide, of the cases I’ve collected of child homicide, 41 per cent were killed by the biological mother, 31 per cent by the biological father and 21 per cent were killed by the de facto male partner.

"The Australian Bureau of Statistics estimates that four per cent of families comprise a step parent and a biological parent, and yet defacto males are responsible for approximately one in five child homicides.

"Step parents don’t have the same level of nurturing or protection as biological parents do."

De Bortoli, of the Centre for Forensic Behavioural Sciences Australia, will publish her findings later this year in a doctorate study she has made of hundreds of child deaths and children's court cases.

She listed five main reasons for child homicide:

Altruistic, when the child is sick or in pain and the parent wants to relieve the child’s suffering;

Acutely psychotic, when a parent is severely psychiatrically ill and the parent kills the child for no other reason;

Spousal revenge, where a parent kills a child to hurt the other parent;

Abusive or accidental, involves ongoing abuse which eventually leads to the child’s death during a very stressful period;

Unwanted children, when a child is rejected or regarded as a hindrance.

"Cases of unwanted children are complex, and the situation may be exacerbated in cases where the mother has a new partner," De Bortoli said.

"Younger children are at increased risk because they are physically more vulnerable and unable to protect themselves. Also, if they are less than four years old, they spend more time at home and so there is greater opportunity for abuse."

A report by the Child Death and Serious Injury Review Committee of South Australia found that almost twice the number of male children compared with female children died, and children younger than four accounted for the majority, 64 per cent, of all deaths from fatal assault or neglect.

The report found "children who live in poverty ... are more likely to be at risk of poorer health and wellbeing.

"Children who lived in areas of greatest disadvantage had a higher rate of death than those who lived in areas of least disadvantage, (with) 16 deaths per 100,000 children in areas of greatest disadvantage compared to 2.6 deaths per 100,000 children in areas of least disadvantage," the report said.

De Bortoli said young male children might be at greater risk of violence because "little boys may be perceived to be more assertive or louder".

"But little children in general may be at increased risk because of the developmental challenges they pose, such as, temper tantrums, teething and toileting and therefore, they are more likely to elicit violent responses from the caregiver."

Cecil Smith knows he will never really understand what led to the fatal bashing of his son Jordan on a hot February night 13 years ago.

The night of February 25 was so warm, baby Jordan wore only a disposable nappy as his mother partied.

Louise Anderson, Hoerler and a group of young adults drank, smoked joints and played with a Ouija board in the suburban brick home Anderson shared with her de facto in a housing commission area of Wagga Wagga.

Her relationship with Cecil Smith had broken down not long after Jordan's birth, and she took up with Hoerler, a former schoolmate of Smith's.

Hours after the party guests had left, Hoerler raised the alarm about Jordan, saying the baby wasn't breathing.

Jordan was lying on the floor of the lounge room and had a red, brown and white substance around his mouth and a bruise on his temple.

His body was still warm. Blood was spattered on the wall of the main bedroom, the baby's cot sheet and on Hoerler's blue Nike shorts. Louise Anderson said later she had only woken when Hoerler cried for help.

The team of Wagga Wagga detectives who investigated the killing found it difficult to come to terms with the brutality of the assault. Detective Inspector Kerry Lewis, who now works in Sydney, said she still thought about the case.

"The injuries were so horrible and to such a young child. It's the worst case I ever dealt with," she said.

Cecil Smith said it was only recently that he had put his life back together, with the help of his fiancée and counselling by a psychologist.

Baby Jordan's uncle, Darcy Smith, said his family was reminded every year on the anniversary of Jordan's murder and every May, when Hoerler, now 36, applied for parole. The NSW State Parole Authority refused his latest parole application just this week, but Hoerler is likely to apply again for release next year.

"He should serve his full term," Darcy Smith said. "They are going to deport him [back to Papua New Guinea] when he gets out, but who's going to protect the PNG community, who's going to supervise him?

"Poor little Jordan would have been a teenager now. It's a shame he isn't out there running around."