Page 686 - Makino,Tsutusi.DictionaryOfIntermediateJGrammar

The choice of yo in the above sentences sounds much less persuasive and
informal than the choice of zo. Note that the sentences of [I] are all unac-
ceptable as self-directed sentences (i.e., monologues) but acceptable if they
are other-directed sentences. The second difference between the two sen-
tence-final particles is that yo can be used by both male and female speakers
but zo is used only by male speakers. The third difference is shown in [3]
below. In the sentence-final combination of yo ne (assertion + confirmation)
yo cannot be replaced by zo.
*m*L K 1% *v t
[3] a. @h!3*a)&&b&f L~L~&;~~I: {ah /*?%I],
(Japanese politics has undergone tremendous change, hasn't it?)
1: IiLCL * lj,LTb,
{Ha
b. B$.AcWW~~I:.~& /*?%I,
(Japanese people are concerned about how they appear to the
world, aren't they?)
The fourth and the last difference stems from the fact that zo can be used
only with informal forms as indicated in Formation. In contrast, the particle
yo can be used either with informal or formal forms.
141 a. z~)%c&LB~LTF IA /*TI,
(This book is interesting, you know.

b. H+AT-T ra / *TI,
(He is a Japanese, you know.)
c. &s&z&ga 2, Wkicts r7 a* ra /*TI,
(If you drink too much sake, you'll get ill, you know.)
(+yo (DBJG: 543-47))