A US father facing a murder charge over his toddler son's death in a baking hot car searched websites that advocated a child-free life, a court has heard.

Justin Ross-Harris, of suburban Atlanta in Georgia, appeared for a probable cause hearing, where a judge ruled there was enough evidence to warrant a trial.

Cobb County Police Detective Phil Stoddard testified that evidence suggested Harris (33) was practically leading a double life.

He said Harris looked at websites in the days leading up to the death of his 22-month-old son that advocated not having children.

He also testified that Harris exchanged nude photos with at least six women while his son, Cooper, was strapped in a child-restraint seat in the back of his SUV.

Mr. Stoddard said the "sexting" included images of Harris' exposed genitalia.

Harris' attorney objected to the testimony - saying it had only been used "to publicly shame him", but prosecutors said the evidence suggested Harris wanted a marriage and child-free life.

Mr. Stoddard said Harris also conducted an internet search on "how to survive in prison".

Harris' online activity has become a central focus in the case after a police warrant revealed that both he and his wife, Leanna, had searched "child deaths inside vehicles and what temperature it needs to be for that to occur".

Forgot his son was in the car

The defendant told investigators he had carried out the online research because "he was fearful that this could happen". But prosecutors allege that Harris intentionally left his young son in the car while he went to work on June 18th.

Harris has pleaded not guilty to murder and charges of child cruelty. Alex Hall, a friend of Harris, said Harris talked about how much he loved his son all the time.

He said he, another 'Home Depot' co-worker and Harris had gone to lunch the day the boy died and had planned to go to the movies after work.

The two men later took Harris to his car so he could put a couple of light bulbs he had purchased inside. His defence attorney, Maddox Kilgore, said that showed Harris did not mean to leave the boy there.

He also said Harris sent a text to his wife asking her what time she planned to pick up Cooper from day care.

Leanna Harris has backed her husband, praising him as a "wonderful father" as she spoke at the toddler's funeral on Saturday. On the day Cooper died, Harris told police he watched cartoons with his son.

He said he then went straight to work, forgetting to drop off the boy at day care. Harris told investigators he realised his son was still in the car as he was driving home.

A defence witness testified that Harris appeared extremely upset after he pulled into a strip mall car park and tried to do CPR on the boy.

A medical examiner determined that Cooper died of hyperthermia - a condition that involves the temperature of the body spiking as a result of heat.