Scotland coaches have dished up a few wacky selection ideas down the years, but Scott Johnson was surely breaking new ground at Murrayfield yesterday when he unveiled his team to face Italy based, at least in part, on the hitherto unexplored principle of trichological determinism.

The phrase is probably new to you. In fact, it was to me until I made it up. In essence, it means an individual's character is determined by their hair colour. So blonds are dizzy. Brunettes are brooding. And redheads are angry. Very angry. Hence the choice of Rob Harley to make his first Scotland start at flanker in Saturday's RBS 6 Nations encounter in place of the injured Al Strokosch.

Since the impressively flame-haired Harley happens to be known as one of the most cerebral fellows in the game – on one Glasgow away trip a team official really did spot him with his head buried in a French translation of Dostoyevsky – his off-field demeanour would tend to undermine Johnson's theory.

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But the coach, whose own style might best be described as a sort of salt-and-pepper bouffant, might just be on to something here.

After all, the Romans are believed to have been the originators of hairism against redheads, a prejudice that presumably owes much to the fact their dreams of imperial expansion came up against a brick wall of scary men with see-you-jimmy locks somewhere around Perth a couple of millennia ago. And then, of course, there is Harley's way of playing the game. He could probably explain it in terms of tactical subtleties, game-plans and complex geometry, but the gist of it is that the 22-year-old likes hitting people.

If that isn't an angry man at work then I don't know what is. And he started young. Long before Harley's dreams of playing for Scotland became a reality, he had ditched any thoughts of running in tries from halfway or entrancing the crowds with his dazzling sidesteps. His heroes were the rugby hard men, the blokes who could hit their opponents into the middle of next week.

"The player I loved watching most was Jason White," Harley revealed. "Especially his tackles. He would just go out and smash people. For me, that is probably still the best feeling in rugby. As great as it is to score a try, if you can absolutely smash someone then that is the perfect feeling."

Some context here might be helpful. Harley may have played only 10 minutes of international rugby so far, but the time he was given on the pitch in Scotland's final summer tour match against Samoa in Apia last June was still long enough for him to deliver the winning try in a 17-16 victory. In other words, he knows how to get over the line. But he'd still rather cut you in half.

"It is probably something I have always done," he said. "I have always enjoyed the defensive side of it and the contact. That's what rugby is all about: being as physical as you can. If you are playing mini rugby you just want to get out there, run about and hopefully smash a few people.

"It has probably been that way for me the whole time. For me, would I rather have scored five tries or made five tackles where I have smashed somebody and knocked the ball out? It's probably the tackles."

So the only man in the side who has never previously started a Test for Scotland is a bit of a bruiser? Well, yes, but to think of Harley only in those terms is to misunderstand him entirely.

Sure, he can do all the piledriver stuff, and do it with unsettling relish, but lately he has added layers to his game by doing some rather impressive shifts on the openside flank instead of the blindside he has mostly occupied for Glasgow for the past couple of seasons.

"I think I came to Glasgow as a six or a second row," Harley said. "Because of that you are seen as a certain kind of player. Showing that I can play seven has maybe widened perceptions and people know I can be useful around the park and effective at the breakdown as well. By showing versatility you do not do yourself any harm.

"The lesson from our game against England is that if you can get in first at the breakdown and get quick ball then it makes attacking a lot easier. It is the same in defence if you get in first and slow down their ball, attacking it harder."

For all that his try-scoring cameo against Samoa eight months ago was pretty much the dream debut for Harley, it just wasn't the dream he has cherished since he pitched up at West of Scotland's Burnbrae ground to play mini rugby all those years ago.

His real dream was always about running out at Murrayfield. In essence, he is about to make debut number two. Is he excited? Just a bit. "It is a huge moment," he said firmly. "But I will have to try to use the huge emotion and try not to be overwhelmed by the situation. I want to use the emotion, but keep it under control.

"It was incredible to be told I was getting a start. Then the nerves began, about the match and the performance. It hammers home that you have to do the work during the week. It is so competitive. We were out there this morning, mauling and smashing each other."

Red-mist rugby.

Just how he likes it.