A teenager who was given an absolute discharge despite being found guilty of sexually assaulting a six-year-old girl was granted leniency because he acted out of “inappropriate curiosity” and not for sexual gratification.

During a three-day trial, Dumbarton sheriff court heard that Christopher Daniel, an 18-year-old dental student, had repeatedly touched the child’s vagina over her clothing as she sat on his knee playing on a computer.

Daniel left court with no conviction and without being placed on the sex offenders’ register because of concerns about the consequences for his future career, according to a statement published on Thursday detailing the sheriff’s reasoning.

Sheriff Gerard Sinclair concluded that the offence was “the result of an entirely inappropriate curiosity of an emotionally naive teenager rather than for the purpose of sexual gratification”.

The written ruling went on: “The accused had appeared both noticeably immature and socially awkward, features confirmed by other evidence in the case. It was fortunate that the complainer appeared to have suffered no injury or long-lasting effects.”

The case prompted an outcry after it was first reported by STV News. The girl’s mother, who spoke to the channel again having read the statement, said: “I am absolutely gobsmacked by this. How can he possibly know this? It also seems to be at odds with the charge, which was of sexual assault.

“He also says that my daughter appears not to have suffered any ‘long-lasting effects’. Again, how can he possibly know this? No one has asked me at any point how my daughter is.”

But the sheriff insisted that Daniel had already suffered as a result of the trial. “The accused was a 17-year-old first offender. He had suffered considerable opprobrium, having been temporarily suspended from his university course. Since the matter had come to light, the accused had suffered seizures and had been diagnosed with epilepsy. During the trial he presented as someone who, with appropriate support and guidance, could become a valuable contributor to society.”

He went on: “Any recorded conviction for this offence would have serious consequences in terms of the accused’s future career. On the authorities, this was also a relevant factor in deciding how to deal with the case. Any sentence would mean that he would probably be unable to continue his university course. The notification requirements of the Sexual Offences Act 2003 would automatically apply to a conviction for this offence unless an absolute discharge was granted.”

Describing the choice not to pass sentence on Daniel as a “wholly exceptional decision”, the sheriff insisted that he considered it unlikely that Daniel would appear in court as an accused again.

The girl’s mother told STV that she found the document “troubling”. “The sheriff says that we are not seeking retribution. We don’t want him locked up, but I have always said he should be put on the sex offenders register. As we suspected, the sheriff also confirms that his career was deemed to be more important. So if he had not done as well in his exams at school, he would have been put on the sex offenders register? I think people will be appalled.”

Responding to the published reasoning, Sandy Brindley, chief executive of Scottish Rape Crisis, said: “It’s hard to see any circumstances where absolute discharge is an appropriate sentence for repeated sexual assault of a six-year-old girl.

“There also seems to be a significant disconnect between the sheriff’s view of what the family wanted and the reality. Finally, the sheriff has stated that the assaults had no impact on the girl, but how does he know?”