Personality differences are a bit tricky. The figures I reported here before were close but not quite right. I can't explain it without using equations so the math-phobic should brace themselves.

First of all, personality is not stored as the values 0-10 you see on your screen. Instead it is stored 0-1000 so a Nice of 7 is actually stored as Nice 700. There are a number of activities such as werewolf transformations and encourage actions which change personality in partial points so that Nice 7 you see is actually somewhere between 700 and 799.

The following calculations are made for each personality characteristic and summed together.

If both sims are on the same side of the scale, either both in the range 0-499 or both in the range 501-1000 (note that 500 is excluded from both ranges), then there is an attraction bonus of 7 - (difference / 71). Example: a sim with Active 600 and a sim with Active 800 would get a bonus of 7 - ((800 - 600) / 71) or 7 - 2 = +5. Why 71? So they could keep the maximum bonus/penalty equal to 7.

Now if the sims are not in the same range, and remember 500 is not in any range so automatically falls here, then there is an attraction penalty. This is calculated for each sim as the (difference between the personality rating and 500) / 71. These two numbers are totaled, although the maximum penalty is -7.

So if we have a sim of Outgoing 400 and a sim of Outgoing 700 they are in different ranges. The penalty from the first sim is (500 - 400) / 71 = 1, and from the second sim is (700 - 500) / 71 = 2. The total penalty then is -3.

Confusing isn't it.

If all that just made your eyes glaze over then I can give an approximation for sims who have don't have partial personality points (which is nearly everyone).

For each personality characteristic the two sims have on the same side of the range, here is the attraction bonus based on personality difference:
0.....+7
1.....+6
2.....+5
3.....+3
4.....+2

For each personality characteristic on opposite sides, each sim contributes to the penalty based on actual personality rating. Add the two to get the actual penalty, but if the penalty is greater than -7 (meaning -8 or lower) then it is -7:
0 or 10.....-7
1 or 9.....-5
2 or 8.....-4
3 or 7.....-2
4 or 6.....-1
5.....0

Personality has the smallest effect on chemistry and yet is the hardest factor to understand. Silly EA.