In order to determine how the Love Ball actually works, I performed a few experiments to test various theories. Read below if you want to find out what the experiments were, and feel free to try them out on your own in case you want to see the proof for yourself. If you're not interested, here is what I concluded:
Summary of how the Love Ball works
The Love Ball only gives an 8x multiplier to the catch rate if your pokémon has the same National Pokédex number, but the opposite gender, of the one that you are trying to catch. The Love Ball never gives an 8x multiplier to the catch rate when catching a genderless pokémon.
The Love Ball, in various sources, is said to give an 8x bonus when catching pokémon of the opposite gender. This has led to confusion, where people have thought that it meant that you could get an 8x bonus using any pokémon of the opposite gender, even if they are completely different types. This been proven to be false, and Bulbapedia (among other sources) lists the additional requirement that the pokémon be the same species as the one in the first slot of your party.
But what is meant by "same species?" Does this refer to the actual pokémon type itself (i.e. if you're trying to catch a Caterpie, will you only get the 8x bonus if your first pokémon is a Caterpie of the opposite gender?) or does "same species" instead refer to the text that is listed in a pokémon's species description in the pokédex (like Pidgey and Natu's "Tiny Bird Pokémon")?
My hypothesis is that the former is true: it only gives an 8x bonus if the pokémon that you are catching is the same type as the first one in your party (i.e. Caterpie vs. Caterpie). If the latter is true, then you could get the 8x bonus with some pretty odd combinations, which is why I strongly suspect that species name has nothing to do with whether the Love Ball works or not. For example, Exeggcute and Chansey are both "Egg Pokémon" and yet Exeggcute is from the Plant egg group and Chansey is from the Fairy egg group, meaning that they can't breed, which would go against the whole point of the Love Ball in the first place (presumably to find a pokémon to breed with yours.) Similarly, Espeon and Sunflora are Sun Pokémon, but Espeon is of the Ground group and Sunflora is from the Plant group. There are some other pretty weird combinations, as well.
It would be awesome if you could get the bonus based on the species name, because then you could catch a Latios/Latias and then take it to the fight with the other one after using Enigma Stone, and since they are both Eon Pokémon, if the species name thing actually works, you would get an 8x bonus. (But, as I later found out, it doesn't work that way!)
For the sake of Science™, I have devised an experiment to test my theory.
The Experiment, Part I
Both Qwilfish and Jigglypuff are of the "Balloon Pokémon" species. If my hypothesis is wrong, then the Love Ball WILL give me an 8x bonus if I bring my Qwilfish up against a Jigglypuff of the opposite gender. This means that I won't even have to reduce Jigglypuff's HP or give it a status ailment; I can just throw a Love Ball at it and it should work instantly (Jigglypuff's catch rate gives it a 22.2% chance of being caught with full HP and no status ailments, and eight times that is greater than 100%, meaning it will always work). Therefore, I did the following:
- Bring my Qwilfish to Kanto Route 4
- Wander around in the grass until we encounter a Jigglypuff of the opposite gender to my Qwilfish.
- Throw a Love Ball at Jigglypuff.
- If it fails to catch Jigglypuff, it will prove that my theory is correct.
- If it catches Jigglypuff, try again at least ten times. If it works every single time, then my theory is probably incorrect, and the Love Ball really does work on pokémon based on species name and not actual pokémon type.
After successfully catching two female Jigglypuffs using a Love Ball with my male Qwilfish in the lead, the third Love Ball failed to catch the female Jigglypuff. This proves not only that the species name ("Balloon Pokémon" in the case of Jigglypuff and Qwilfish), has nothing to do with the Love Ball multiplier, but that the Love Ball doesn't work based on gender alone (which we kind of already knew, but it's good to have additional proof.)
The Experiment, Part II
It would be worthwhile to test another hypothesis: maybe the Love Ball will give the 8x bonus if you are catching a pokémon of the same evolutionary chain as the one in your first slot. This means that if I bring an Igglybuff to Kanto Route 4 and use a Love Ball on a Jigglypuff of the opposite gender, it should work instantly. Here are the steps:
- Bring my Igglybuff to Kanto Route 4.
- Wander around in the grass until we encounter a Jigglypuff of the opposite gender to my Igglybuff.
- Throw a Love Ball at Jigglypuff.
- If it fails to catch Jigglypuff, it means that my hypothesis is incorrect, and the Love Ball only works if you're catching a Pokémon of the exact same type.
- If it catches Jigglypuff, try again at least ten times. If it works every single time, then my theory is probably correct, and the Love Ball works on pokémon of the same evolutionary chain.
The first female Jigglypuff that I tried to catch--using a Love Ball with my male Igglybuff in the lead--did not work. This proves that my hypothesis was incorrect, and that being a member of the same evolutionary chain does not affect the Love Ball's multiplier.
The Experiment, Part III
To make sure that the failure of the Love Ball in the above experiment was not a result of Igglybuff being a member of the no eggs group, I took a male Slowking to the Slowpoke Well and attempted to use a Love Ball on a female Slowpoke.
The first time I tried a Love Ball on a female Slowpoke while my male Slowking was in the lead, it failed. This proves that being an opposite gender member of the same evolutionary chain does not affect the Love Ball's multiplier.
As stated above, the Love Ball only has an 8x multiplier if it is used on a Pokémon that is the opposite gender but exact same type as the one being caught. In other words, for the Love Ball to work, you need to bring a pokémon whose National Pokédex number is exactly the same as the one being caught, and whose gender is the opposite of the one being caught.