Weird spacing between some letters in math mode

Question

Input:

\begin{equation}
\begin{split}
democ_{i,t}={} & \alpha\,democ_{i,t-5} \quad+\quad 
\beta\,students\;abroad_{i,t-5} \quad+\quad \\ & 
\gamma\,democ.\;in\;receiving\;countries_{i,t-5} \quad+\quad\ \\ & 
\delta\,(students\;abroad_{i,t- 
5}*democ.\;in\;receiving\;countries_{i,t-5}) \quad+\quad \\ & 
country\;fixed\;effects_i \quad+\quad time\;fixed\;effects_t 
\quad+\quad \epsilon_{i,t}
\end{split}
\end{equation}

Output: Why is there extra spacing between the f's in 'effects' when every other word is properly spaced? How do I fix it?

Answer 1

I propose this format (with \bigl( \bigr) if you wish):

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}
\textit{democ}_{i,t} &= 
\alpha\, (\textit{democ}_{i,t-5}) + \beta\, (\textit{students abroad}_{i,t-5}) \\ 
&\quad  + \gamma\, (\textit{democ. in receiving countries}_{i,t-5}) \\ 
&\quad  + \delta\, (\textit{students abroad}_{i,t- 5}) (\textit{democ. in receiving countries}_{i,t-5}) \\ 
&\quad  + \textit{country fixed effects}_i +  \textit{time fixed effects}_t + \epsilon_{i,t}
\end{aligned}
\end{equation}
\end{document}

Answer 2

Inspired by Mico's solution, here's an improved one that

1. Doesn't need \,
2. Automatically adds parentheses if the variable consists of several words
3. Uses \mathit instead of \textit
4. Doesn't need “backslash space” after a period

Here's the code

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\vn}{m}
 {
   \seq_set_split:Nnn \l_tmpa_seq { ~ } { #1 }
   \int_compare:nTF { \seq_count:N \l_tmpa_seq > 1 }
    {
     \seq_set_map:NNn \l_tmpb_seq \l_tmpa_seq { \exp_not:N \mathit { ##1 } }
     (\seq_use:Nn \l_tmpb_seq { \  })
    }
    { \mathop{}\!\mathit{#1} }
 }
\ExplSyntaxOff

\begin{document}

\begin{equation}
\begin{split}
\vn{democ}_{i,t} ={}
& \alpha \vn{democ}_{i,t-5} + \beta \vn{students abroad}_{i,t-5} \\
& +\gamma \vn{democ. in receiving countries}_{i,t-5} \\
& +\delta \vn{students abroad}_{i,t-5} \vn{democ. in receiving countries}_{i,t-5} \\ 
& +\vn{country fixed effects}_i + \vn{time fixed effects}_t + \epsilon_{i,t}
\end{split}
\end{equation}

\end{document}

Answer 3

Here's an answer that's quite similar, at first glance, to @Sigur's answer. It differs from Sigur's by (a) providing adjustments to make sure that the . ("dot", "period", "full stop") isn't misinterpreted as a sentence-ending punctuation mark, (b) placing the parentheses solely around the variable names, not their subscript terms, (c) providing for explicit italic corrections and, most importantly, (d) taking a more "LaTeX-y" approach to things, by which I mean separating meaning from form, by defining a macro called \vn (short for "variable name") that's to be used to display variable names. In the code below, I've chosen the definition \newcommand\vn[1]{\textit{#1}}. An advantage of such a setup is that if you ever, at some point in the future, decide that you'd rather display variable names using an upright rather than an italic font, all you'd need to do is change the definition of \vn. In particular, you would not have to go and check each and every instance of \textit in the document and decide whether it needs to be changed to \textrm.

The following code also makes some spacing adjustments between \vn{democ} and its associated subscript terms.

\documentclass{article}
\usepackage{amsmath}
\newcommand\vn[1]{\textit{#1\/}} % to display a variable name
    %% note the italic correction provided by "\/"
\begin{document}

\begin{equation}
\begin{split}
\vn{democ}_{\mkern1.5mu i,t} &= 
\alpha\, \vn{democ}_{\mkern1.5mu i,t-5} + \beta\, (\vn{students abroad})_{i,t-5} \\ 
&\quad +\gamma\, (\vn{democ.\ in receiving countries})_{i,t-5} \\ 
&\quad +\delta\, (\vn{students abroad})_{i,t-5}\, (\vn{democ.\ in receiving countries})_{i,t-5} \\ 
&\quad +(\vn{country fixed effects})_i + (\vn{time fixed effects})_t + \epsilon_{i,t}
\end{split}
\end{equation}
\end{document}