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It's hard to separate the effects of the posters from the larger effects of mass propaganda, and of using shame as a strategy in general, most notably the notorious White Feather campaign.
But the latter's effect is usually seen as effective in the short term, boosting enlistment numbers. Views of the campaign subsequently turned negative, and it wasn't so prevalent during WWII. I suspect that's what the "negative effect" is referring to.
The campaign is where young women were encouraged to hand out white feathers - symbols of cowardice - to healthy unenlisted men, in order to shame them into enlisting. Due to gender roles and the view of masculinity at the time, this was effective and helped increase enlistment, but the community quickly took a dim view of the campaign and the women who participated in it, especially when the recipients of the feathers were undeserving, such as discharged servicemen or those on leave, and adolescents.
For more info, see The White Feather Campaign: A Struggle with Masculinity During World War I