Neither sister had an alibi for the time of the murders. Their parents’ house was searched for fingerprints, and none belonging to any strangers were found. The elderly couple had become very reclusive in their old age and had made a will that divided their estate equally between their two daughters, who both earned good salaries. Despite this, the elder daughter, who loved shopping and partying, was in considerable debt, while the younger sister had accumulated significant savings.

A week after the murders, the police reviewed their notes on the case. They had no leads and could prove nothing. Reluctant to give up, one of the detectives had an idea. ‘It’s got to be one of the daughters,’ he said. ‘No one else benefited from these deaths.’

His colleagues agreed the killer was likely to be one of the two daughters.

‘I’ve got an idea, then,’ said the detective. ‘Let’s tell each sister that we have incontrovertible proof that she’s the killer. Continue to claim innocence, we’ll say, and that will count against you when it comes to sentencing. But admit to your crime and you’ll be treated more leniently. We say this to both of them, and see if either one breaks down and tells us the truth.’

One detective objected. ‘We should only say all that to the one who killed her parents,’ she said. ‘Only one of those two sisters could conceivably be guilty of murder.’

Everyone agreed she had a point. ‘All the same,’ said one officer, ‘I’d be happier if we dangled the bait before both sisters. I mean, I agree it’s super unlikely that the younger sister suddenly decided to become a murderer—’

The detective who’d intervened held up her hand. ‘Did you think I was referring to the older sister when I mentioned the only one who could conceivably be guilty?’

All the detectives in the room nodded.

‘Hell, no,’ she said. ‘The opposite. The elder sister is selfish and hedonistic, but she can’t bear to spend a second of her life not having fun. She’s almost allergic to the absence of pleasure. Now, you’re going to say that she was in debt and needed her inheritance, whereas the younger sister didn’t, but the older sister didn’t care about debt. I doubt she ever gave a thought to how much she owed. She’ll have buried her head in the sand and pretended everything was fine, because she could only bear to exist in a world that contained happy, fun things and no depressing realities.’

‘Whereas the younger sister—’ one detective began, as light started to dawn.

‘Yes. The younger sister worries about money even though she’s comfortably off. That’s why she saves: too much. A single woman, earning what she earns, with disproportionately large savings – that’s someone who worries about money.’