The evidence that a songwriter working alone is perfectly capable of writing a hit does exist but the case for collaboration is substantial, especially in certain genres such as country.
While both situations have positives and both have negatives it's important to note that far more "outside" hits are written by songwriters collaborating with others than by lone songwriters. That would suggest that yes, two, three or even more heads are better than one.
An "outside song" is one written by a songwriter or songwriters with little or no connection directly to the artist its later pitched to. While the particulars differ, an outside song usually has a gauntlet of "gatekeepers" to negotiate before the artist ever hears it...A&R types, publishers, and others who can potentially reject it and stop the song from progressing any further in that direction.
An inside song is one written either by the artist or someone in the artist's camp so that song has an inside track to getting on the project. It doesn't have to run the gatekeeper gauntlet.
So consider the positives and negatives before deciding whether to go it alone or write with others.
Collaboration Positives:
1. Companionship for those who hate working alone.
2. Building your connections network
3. More than one person marketing the song after its finished.
4. Splitting of demo expenses.
Collaboration Negatives:
1. Royalty streams are split.
2. Additional paperwork.
3, Potential disagreements, from which elements are included to the use of the song.
4. Potential stress if things start going in a negative direction.
5. If you self manage the song, additional management time long term.
6. Working by appointment rather than when the mood strikes.
It may be uncomfortable to suggest signing a Collaboration Agreement upfront, but its definitely the way to go. It establishes when the song was started and who was part of the writing process. Ideally it states who is responsible for what expenses, how royalties will be split. and what happens to the split if a new writer is added later.
Why would that even happen? Why add a writer after the song is finished?
Songwriters without the clout and reputation of a hit may submit a song to a major label artist's camp and the artist might love the song but may desire a piece of the songwriting action. Basically you get a choice: Add the artist as a songwriter even though they wrote not a word nor a note, or walk. Fair? That depends on who you ask. But most songwriters will accept that deal and suddenly you have added one or more collaborators.
If there are only two of you in the original collaboration and that deal is offered, what if you disagree on accepting or declining? A good Collaboration Agreement will address those types of issues. It could be as simple as a blanket statement stating that if you both disagree, you flip a coin to decide.
Another reason a collaborator joins later? A foreign version needs to be created and the new foreign lyric writer wants a piece of the pie on the altered version.
It may be the status quo and much more fun to just dive in and start writing but it is highly inadvisable to rely on memory. Yes, you just spent hours together creating this masterpiece, who could forget, right? But fifteen years and 150 collaborations down the road all those writing sessions will start to meld together in your mind like fruit in a smoothie blender...what was the name of the woman who helped write that country two step back in 2021...or wait, was she even there? Maybe that was a different song. And who wrote that great line about the eagle in the sky...I think that was Jason Smi...er no, maybe it was.. man, write it down, have all parties sign it.