IF THERE IS ONE LINE in Shakespeare that is more underrated than any other it is the four words uttered by Macduff when he has just been told that Macbeth has had his children murdered. But the would-be king Malcolm is urging him to think only of revenge.

“He has no children.” There it is: the great gulf in humanity. In a nutshell the impossibility of explaining to the childless the heart-stopping, soul-tugging moment of almost blind panic experienced by the parent who fears mortal harm to a child.

Is there anything easier to exploit? Any weakness more apparent? Any faultline in the human psyche easier to fracture? Michael Robotham clearly doesn't think so. When the psychologist Joseph O'Loughlin is called from his part-time lecturing class by police seeking his help in preventing a woman leaping from the Clifton Suspension Bridge, he has no idea what is in her mind.

He has even less idea why, as she stands there naked save for red Jimmy Choo shoes, and with the word “slut” scrawled across her torso in lipstick, she is still talking into her mobile phone. Until it becomes clear that the woman will do whatever the voice on the other end tells her, even when that voice says: “Jump!” When a few days later her friend and business partner ends up hanging from a tree in a park dead of hypothermia and wearing only handcuffs, he begins to suspect a pattern. Someone is making these women kill themselves, and the only reason he can imagine is that they think the killer is making a deal: their life for their child's.