Global variables in Handlebars if blocks

Is it possibly to use global variables in Handlebars conditionals? I'm writing an app that lists a lot of objects, and I want users to be able to control which details are listed. For example, displaying only first names in a list of people, like so:

<ul>
{{#each people}}
    <li>
        <p>{{firstName}}</p>
        {{#if displayLastnames}}
        <p>{{lastName}}</p>
        {{/if}}
    </li>
{{/each}}
</ul>

I don't want to actually modify the data (for example, by removing the lastName attribute and doing {{#if lastName}}).