Oh right. Most of my experience is with the Space-Empires line, which doesn't have such, beyond perhaps weapons, although I suppose with the modifiability of the game...
Look at it this way:
If I researched Photon Pulse Weapons (whatever they are...) then it will cost me less to develop Photon Torpedo Weapons because I already have much of the basic knowledge.
On the other hand if I develop technologies that use Neutrinos (a sub-atomic particle) It will cost me a lot lot more to research technologies using Anti-Neutrinos because if they aren't separated properly they would annihilate, rendering BOTH devices useless.
In fact it may not even be possible to use both in one civilisation therefore if I have one I CANNOT research the other at all.
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