I wonder whether there’s an expiry date on sitcoms. As any series gets older it becomes harder to keep things fresh, while not making so many changes that the series becomes unrecognisable. As the years pass it becomes increasingly unlikely that the characters would all still be in the same place, the same jobs, the same friendship groups, the same domestic situations and more tellingly, making the same mistakes. Many dramas get stuck in a rut of endless running through the same procedures with different guest stars and window dressing, but with a comedy you’ve got the added pressure of making the whole thing funny.

What’s strange is that when I started writing this review, I was all set for a lament about how that applies to The Big Bang Theory, but now as I come to document the evidence, I’m wondering if maybe the complete opposite is true. Actually, The Big Bang Theory HAS managed to avoid many of the common pitfalls while still staying true to its original pitch.

The addition of Amy and Bernadette, and even the tentative inclussion of Stuart the Comic Shop Guy, and recurring characters like Kripke and Lucy, have broadened the potential of the show. While it’s true that the characters are broadly in the same jobs and same situations, and the friendships between the group haven’t changed much, the romantic relationships have and each of the pairings have matured satisfyingly. While characters do tend to take 2 steps forward and 1 step back, they are at least still making steady, if slow progress forwards.

The moments in the show that demonstrate character growth are really lovely – Sheldon and Amy moving forward in their relationship, Penny telling Leonard she loves him, Raj talking to Penny, the less girls playing Dungeons and Dragons, or discussing comic books to better understand the guys. Each of those moments proves that not only can the characters age and grow, but they can still be funny while they do it. They’re moments that I re-watch and chat about with friends, and they’re timelessly endearing.

That all sounds great, so why was my first instinct to launch this review with talk about expiry dates? The problem is I think that while the memories that stick are the real highlights of the show, when I watch week to week I’m overwhelmed by the frustrations that leave me wanting to hide behind cushion. Raj is a particularly difficult character, with his jokes about money and India feeling increasingly uncomfortable. Sheldon has made some big steps forward (for him), but it’s the leaps backwards where he comes across as downright mean and inappropriate (the whole HR storyline).

For the moment, there’s enough there to keep me coming back for the next season. It will be interesting to see how the group have coped without Leonard for a few months and that should provide some great material, allowing the show to investigate what it’s like without one of the characters, without actually being without them. For now the potential and the good outweigh the short term frustrations, and this is still the only comedy that I really watch. So fingers crossed they keep moving in the right direction, even if it is very slowly.