To me, and based on dictionary definitions a **"determinative factor"** and a **"decisive factor"** mean exactly the same thing, while they imply a very "crucial factor" which can lead  you to make a decision about something.

Also, I think I can use the noun "determinant" instead of "a determinative factor": <br>

**Determinant:** <br>

> Something that controls or affects what happens in a particular situation: <br>

- Soil and climate are the main **determinants** of how land is used. <br>

**Determinative:** <br>

> 1. able to or serving to settle or determine; deciding <br>
    2. a factor, circumstance, etc. that settles or determines <br>

- A **determinative factor** influences that are determinative of future behavior.

**Decisive:** <br>
> If a fact, action, or event is decisive, it makes it certain that there will be a particular result. "Strongly" affecting how a situation will progress or end. An action, event etc that is decisive has a big effect on the way that something develops; a "decisive factor"/effect/influence etc.

- The economy is regarded as the "decisive factor" which will determine the outcome of the general election.

I wonder if you could tell me more about these two terms.